Old bachelors engineered goal : it’s essential to the score (9)
I believe the answer is:
obbligato
'essential to the score' is the definition.
(musical term describing an essential instrument)
'old bachelors engineered goal it's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bachelors' becomes 'bb' (I can't justify this - if you can you should give a lot more credence to this answer).
'engineered' is an anagram indicator (I've seen this in other clues).
'goal'+'it'='goalit'
'goalit' anagrammed gives 'ligato'.
'o'+'bb'+'ligato'='OBBLIGATO'
(Other definitions for obbligato that I've seen before include "Instrumental part not to be omitted" , "important passage of music" , "Essential part of a piece of music" , "vital part of performance?" , "Musical accompaniment" .)