Former husband quiet with increase (6)
I believe the answer is:
expand
'increase' is the definition.
(expanding is a kind of increasing)
'former husband quiet with' is the wordplay.
'former husband' becomes 'ex'.
'quiet' becomes 'p' (music symbol for 'piano').
'with' becomes 'and' (I've seen this before).
'ex'+'p'+'and'='EXPAND'
(Other definitions for expand that I've seen before include "Extend in one or more directions" , "Spread out" , "Grow larger, like business" , "Distend" , "Make larger" .)