Complete start of form badly (4)
I believe the answer is:
fill
'complete' is the definition.
(thesaurus)
'start of form badly' is the wordplay.
'start of' indicates taking the first letters.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
The first letter of 'form' is 'f'.
'f'+'ill'='FILL'
(Other definitions for fill that I've seen before include "Occupy the whole of" , "Top up" , "` `Work expands to .... the time available' '" , "Replenish" , "Appoint person to (post)" .)