A sticker brought into play, nice one! (7)
I believe the answer is:
attaboy
'nice one' is the definition.
(interjections!)
'a sticker brought into play' is the wordplay.
'sticker' becomes 'tab' (small piece of paper for identification).
'brought into' indicates putting letters inside.
'play' becomes 'toy' (synonyms).
'tab' going within 'toy' is 'ttaboy'.
'a'+'ttaboy'='ATTABOY'
(Other definitions for attaboy that I've seen before include "What's said to encourage" , "word of encouragement" , "Come on!" , "Expression of encouragement, Tay boat (anag.)" .)