Service uniform, good worn by lad (8)
I believe the answer is:
evensong
'service' is the definition.
(thesaurus)
'uniform good worn by lad' is the wordplay.
'uniform' becomes 'even' (I've seen this before).
'good' becomes 'g' (abbreviation).
'worn by' is an insertion indicator (I've seen this in other clues).
'lad' becomes 'son'.
'even'+'g'='eveng'
'eveng' enclosing 'son' is 'EVENSONG'.
(Other definitions for evensong that I've seen before include "6 [SERVICE] ?" , "Late afternoon church service" , "Anglican service" , "Religious service" , "Vespers" .)