Steep area close to summit guarded by a soldier (8)
I believe the answer is:
marinate
'steep' is the definition.
(marinating is a kind of steeping)
'area close to summit guarded by a soldier' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'close to' suggests the final letters (the close/ending of).
'guarded by' is an insertion indicator.
'a soldier' becomes 'marine' (I've seen this before**).
The final letter of 'summit' is 't'.
'a'+'t'='at'
'at' inserted inside 'marine' is 'MARINATE'.
(Other definitions for marinate that I've seen before include "Soak meat, fish etc" , "Soak meat in seasoned liquid before cooking" , "Soak or steep meat or fish" , "Soak in a mixture of ingredients before cooking" , "Soak in spiced liquid" .)