Try a lure, catching tons (7)
I believe the answer is:
attempt
'try' is the definition.
('attempt' can be a synonym of 'try')
'a lure catching tons' is the wordplay.
'lure' becomes 'tempt' ('tempt' can be a synonym of 'lure'**).
'catching' indicates putting letters inside.
'tons' becomes 't'.
'tempt' enclosing 't' is 'ttempt'.
'a'+'ttempt'='ATTEMPT'
(Other definitions for attempt that I've seen before include "Try, endeavour" , "Have a crack" , "Tackle" , "bash" , "Stab" .)