Delay one in enclosure (4)
I believe the answer is:
stay
'delay' is the definition.
(I've seen this before)
'one in enclosure' is the wordplay.
'one' becomes 'a'.
'in' is an insertion indicator.
'enclosure' becomes 'sty' (sty is a kind of enclosure).
'a' put within 'sty' is 'STAY'.
(Other definitions for stay that I've seen before include "Remain; suspend" , "guy giving > support" , "remain in the same place" , "Remain; prop" , "Thin strip used to stiffen a garment" .)