Yield to pressure of mounting wind (3)
I believe the answer is:
sag
'yield to pressure' is the definition.
(I've seen this in another clue)
'mounting wind' is the wordplay.
'mounting' shows that the letters should be reversed in order (going up in a down clue).
'wind' becomes 'gas' (synonyms).
'gas' back-to-front is 'SAG'.
'of' acts as a link.
(Other definitions for sag that I've seen before include "Decline" , "Drop" , "Droop, sink in the middle" , "Sinking" , "Hang down loosely" .)