Upgrade rear of battle tank in shelter (7)
I believe the answer is:
elevate
'upgrade' is the definition.
(I know that upgrade can be written as elevate)
'rear of battle tank in shelter' is the wordplay.
'rear of' indicates one should take the final letters.
'tank' becomes 'vat' (type of container).
'in' indicates putting letters inside.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
The final letter of 'battle' is 'e'.
'vat' inserted into 'lee' is 'levate'.
'e'+'levate'='ELEVATE'
(Other definitions for elevate that I've seen before include "Lift up or ennoble" , "Improve morally" , "Hoist" , "Raise or lift" , "'Raise, upgrade (7)'" .)