It's engaged setter and solver in past (4)
I believe the answer is:
busy
'it's engaged' is the definition.
('busy' can be similar in meaning to 'engaged')
'setter and solver in past' is the wordplay.
'setter and solver' becomes 'us' (the setter and the solver of the crossword).
'in' indicates putting letters inside.
'past' becomes 'by' ('by' can be a synonym of 'past').
'us' placed into 'by' is 'BUSY'.
(Other definitions for busy that I've seen before include "Very active; engaged" , "Fully employed" , "Active, occupied" , "Active (as a bee?)" , "Like a bee?" .)