Husband switches seat in rush (5)
I believe the answer is:
haste
'rush' is the definition.
(I know that rush can be written as haste)
'husband switches seat in' is the wordplay.
'husband' becomes 'h' (abbreviation).
'switches' indicates an anagram (I've seen 'switching' mean this).
'in' is an insertion indicator.
'seat' anagrammed gives 'aste'.
'h' put inside 'aste' is 'HASTE'.
(Other definitions for haste that I've seen before include "Over-eager speed" , "Speediness or rush" , "Urgent hurry" , "Leader abandons modest" , "More of this proverbially slows you down" .)