Clavícula translates to "collarbone". It's an interesting and somewhat unexpected word to find in a song, making it memorable and unique.
In the lyrics, "El frío entrando en mi clavícula" (The cold entering my collarbone) evokes a very physical and visceral sensation. It's a poetic way to express a deep, penetrating feeling of cold, which often symbolizes loneliness or a harsh reality in music. This specific imagery makes the word stand out.
Diego Torres opens "Mirar Atrás" already lost: "Ya no sé quien soy / Vivo en tu dolor" (I don't know who I am anymore / I live in your pain). He is still praying — "Rezo por tu amor" (I pray for your love) — while saying out loud that he thinks this is the end. The chorus does not ask for the relationship back; it asks for a vantage point. "Quiero mirar atrás / Para decirte si al final / Todo lo que fue será" (I want to look back / to tell you if in the end / all that was will be). The verb he leans on, será, is just the future of ser, "it will be" — and he repeats it until it stops sounding like a prediction and starts sounding like a way of setting the question down.
La Mala Rodríguez takes the middle verse and drags the same question to street level. "Seguimos siendo barro, un mismo hijo, un mismo tarro" (we are still mud, the same child, the same jar) — nothing about these two is special, and that is almost the comfort. Her sharpest line is physical: "El frío entrando en mi clavícula / Me recuerda que a veces es mejor decir que no" (the cold coming into my collarbone / reminds me that sometimes it is better to say no). By the last verse Torres has stopped arguing and started listing what the future might turn out to be — "Será tu cruz el miedo / Será mi luz, tu sueño" (fear will be your cross / my light will be your dream) — before landing on "Tal vez será el amor" (maybe it will be love).