Constant pants moving around a windlass (7)
I believe the answer is:
capstan
'windlass' is the definition.
(capstan is a kind of windlass)
'constant pants moving around a' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'moving' indicates an anagram.
'around' means one lot of letters goes inside another.
'pants' anagrammed gives 'pstan'.
'c'+'pstan'='cpstan'
'cpstan' going around 'a' is 'CAPSTAN'.
(Other definitions for capstan that I've seen before include "Dockside rope winder" , "Device winding in rope" , "Cylinder for winding ropes/cables" , "item of winding gear" , "Turner" .)