Remain without a pen (3)
I believe the answer is:
sty
'pen' is the definition.
(both can mean an animal enclosure)
'remain without a' is the wordplay.
'remain' becomes 'stay' (synonyms).
'without' indicates named letters should be taken away.
'stay' with 'a' removed is 'STY'.
(Other definitions for sty that I've seen before include "Pig pen" , "Enclosure for swine" , "Pig-house" , "Pig home or eye sore" , "See 15" .)