Want pressure let out (6)
I believe the answer is:
please
'want' is the definition.
(I've seen this before)
'pressure let out' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'let out' becomes 'lease' (I've seen this in another clue).
'p'+'lease'='PLEASE'
(Other definitions for please that I've seen before include "Cause (someone) to feel satisfied" , "Beatles song (request to oblige?)" , "See 10 Across" , "Elapse (anag.)" , "Like" .)