Irritation to be under husband’s thumb (5)
I believe the answer is:
hitch
'thumb' is the definition.
(I know that thumb can be written as hitch)
'irritation to be under husband's' is the wordplay.
'irritation' becomes 'itch' (an itch irritates you).
'to be under' means one lot of letters go next to another (below, in a down clue).
'husband' becomes 'h' (abbreviation).
'itch' put after 'h' is 'HITCH'.
(Other definitions for hitch that I've seen before include "Snag - free ride" , "Temporary difficulty" , "get a free ride" , "couple" , "Jerk" .)