Keep an article in an enclosure (4)
I believe the answer is:
stay
'keep' is the definition.
(I've seen this in another clue)
'an article in an enclosure' is the wordplay.
'an article' becomes 'a' (the indefinite article in English).
'in' indicates putting letters inside.
'an 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 "Supporting rod" , "Corset strut" , "Remain; brace" , "Support - postponement" , "Remain; steady" .)