Hence, fears grow off and on (4)
I believe the answer is:
ergo
'hence' is the definition.
(I've seen this before)
'fears grow off and on' is the wordplay.
'off and on' means one should take alternating letters (letters taken on and off, on a regular basis).
'fears'+'grow'='fearsgrow'
The alternating letters of 'fearsgrow' are 'ERGO'.
(Other definitions for ergo that I've seen before include "'Cogito, ---- sum'" , "Therefore (Latin)" , "Consequently" , "It follows" , "Thus" .)