Volunteers pen - that's nice (5)
I believe the answer is:
tasty
'nice' is the definition.
(thesaurus)
'volunteers pen' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'pen' becomes 'sty' (both can mean an animal enclosure).
'ta'+'sty'='TASTY'
'that's' is the link.
(Other definitions for tasty that I've seen before include "Like nice grub" , "Appetising, having pleasant flavour" , "Scrumptious" , "Mouth-watering" , "Having good flavour" .)