Orderly volunteers nurse for retirement (4)
I believe the answer is:
neat
'orderly' is the definition.
(I know that orderly can be written as neat)
'volunteers nurse for retirement' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'for retirement' says the letters should be written backwards (retire can mean to retreat or go backwards).
'ta'+'en'='taen'
'taen' in reverse letter order is 'NEAT'.
(Other definitions for neat that I've seen before include "Tidy and spruce" , "Taken without water" , "pat?" , "avoiding water?" , "Cool - without a mixer" .)