Suit line adopted by boys in uniform (5)
I believe the answer is:
clubs
'suit' is the definition.
(I know that clubs is an suit)
'line adopted by boys in uniform' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'adopted by' is an insertion indicator (I've seen this in other clues).
'boys in uniform' becomes 'cubs' (I can't justify this - if you can you should give a lot more credence to this answer).
'l' inserted inside 'cubs' is 'CLUBS'.
Can you help me to learn more?
(Other definitions for clubs that I've seen before include "woods, for example" , "Orders" , "Card suit" , "Suit (in cards)" , "For game, you may need five" .)