Pressure husband coming in to assist sucker (5)
I believe the answer is:
aphid
'sucker' is the definition.
(I've seen this before)
'pressure husband coming in to assist' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'husband' becomes 'h'.
'coming in to' indicates putting letters inside.
'assist' becomes 'aid' ('aid' can be a synonym of 'assist').
'p'+'h'='ph'
'ph' put inside 'aid' is 'APHID'.
(Other definitions for aphid that I've seen before include "Small insect like greenfly" , "Horticultural pest" , "Garden pest" , "Garden bug" , "Fly" .)