President mostly gets tense behind cleaner (5)
I believe the answer is:
bidet
'behind cleaner' is the definition.
(I've seen this before)
'president mostly gets tense' is the wordplay.
'president' becomes 'biden' (Joe Biden).
'mostly' means to remove the last letter.
'gets' is a charade indicator (letters next to each other).
'tense' becomes 't' (abbreviation used in many dictionaries).
'biden' with its last letter taken away is 'bide'.
'bide'+'t'='BIDET'
(Other definitions for bidet that I've seen before include "behind cleaner" , "Stay behind" , "bathroom cleaner" , "Bathroom fitting" , "Bathroom facility" .)