Prisoner to wait for nurse (8)
I believe the answer is:
conserve
'nurse' is the definition.
(as in nursing a beer)
'prisoner to wait' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'to wait' becomes 'serve' (to wait on someone is to serve them).
'con'+'serve'='CONSERVE'
'for' acts as a link.
(Other definitions for conserve that I've seen before include "Save (for later use)" , "Save (eg energy)" , "Protect from harm; jam" , "Save - jam with whole pieces of fruit" , "Use cautiously and frugally" .)