Hold back if in on act being planned (7)
I believe the answer is:
contain
'hold back' is the definition.
('contain' can be a synonym of 'hold back')
'in on act being planned' is the wordplay.
'being planned' indicates anagramming the letters.
'in'+'on'+'act'='inonact'
'inonact' anagrammed gives 'CONTAIN'.
'if' acts as a link.
(Other definitions for contain that I've seen before include "I cannot (anag) - hold back" , "Hold within, suppress (7)" , "Control, restrain" , "keep within bounds" , "Keep within limits or have within" .)