Robust rule observed in musical composition (6)
I believe the answer is:
sturdy
'robust' is the definition.
('sturdy' can be similar in meaning to 'robust')
'rule observed in musical composition' is the wordplay.
'rule' becomes 'r' (legal abbreviation).
'observed in' is an insertion indicator.
'musical composition' becomes 'study' (study is a kind of musical composition).
'r' inserted into 'study' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Stout, brawny" , "Strong and reliable" , "Powerful" , "Stalwart, stout, tough" , "Strong and stout" .)