Amateur entered activity late (7)
I believe the answer is:
delayed
'late' is the definition.
(thesaurus)
'amateur entered activity' is the wordplay.
'amateur' becomes 'lay' ('lay' can be similar in meaning to 'amateur').
'entered' indicates putting letters inside.
'activity' becomes 'deed' (I've seen this in other clues).
'lay' going within 'deed' is 'DELAYED'.
(Other definitions for delayed that I've seen before include "Held up, back" , "Detained, help up" , "Hindered" , "Shelved" , "Protracted" .)