Withdrew sappers in need of a rest (7)
I believe the answer is:
retired
'withdrew' is the definition.
('retire' can be a synonym of 'withdraw')
'sappers in need of a rest' is the wordplay.
'sappers' becomes 'RE' (Royal Engineers).
'in need of' says to put letters next to each other.
'a rest' becomes 'tired' (I can't justify this - if you can you should give a lot more credence to this answer).
're'+'tired'='RETIRED'
Can you help me to learn more?
(Other definitions for retired that I've seen before include "Stopped work - went to bed" , "Withdrew from active participation" , "went back" , "Withdrawn from active participation" , "Gave up job, relaxed on pension" .)