Whole group on return taking in army (5)
I believe the answer is:
total
'whole' is the definition.
(total is a kind of whole)
'group on return taking in army' is the wordplay.
'group' becomes 'lot' (lot is a kind of group**).
'on return' shows that the letters should be reversed in order.
'taking in' indicates putting letters inside.
'army' becomes 'ta' (abbreviation for Territorial Army).
'lot' written backwards gives 'tol'.
'tol' enclosing 'ta' is 'TOTAL'.
(Other definitions for total that I've seen before include "Not qualified" , "gross" , "Count" , "The whole, sum" , "Whole, complete" .)