Working with gear in top form (9)
I believe the answer is:
operating
'working' is the definition.
('operate' can be a synonym of 'work')
'gear in top form' is the wordplay.
'form' is an anagram indicator (another form of the letters).
'gear'+'in'+'top'='gearintop'
'gearintop' with letters rearranged gives 'OPERATING'.
'with' acts as a link.
(Other definitions for operating that I've seen before include "Cutting" , "Directing or controlling, machinery say" , "at work" , "(Of machine) running" , "Active" .)