Remain leader of animals in pen (4)
I believe the answer is:
stay
'remain' is the definition.
(I know that remain can be written as stay)
'leader of animals in pen' is the wordplay.
'leader of' suggests taking the first letters.
'in' indicates putting letters inside.
'pen' becomes 'sty' (both can mean an animal enclosure).
The first letter of 'animals' is 'a'.
'a' going into 'sty' is 'STAY'.
(Other definitions for stay that I've seen before include "Corset strut" , "Remain; corset part" , "Defer" , "Reside" , "Temporary residence" .)