Man in army corps climbing round back of hill (5)
I believe the answer is:
elmer
'man' is the definition.
(I've seen this before)
'army corps climbing round back of hill' is the wordplay.
'army corps' becomes 'reme' (Royal Electrical and Mechanical Engineers).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'round' means one lot of letters goes inside another.
'back of' indicates one should take the final letters.
The final letter of 'hill' is 'l'.
'reme' back-to-front is 'emer'.
'emer' placed around 'l' is 'ELMER'.
'in' acts as a link.
(Other definitions for elmer that I've seen before include "Man" , "name" , "____ Rice, US dramatist" , "- - Gantry (book, film)" .)