Vanish from area, protected by female military engineer (9)
I believe the answer is:
disappear
'vanish' is the definition.
(synonyms)
'area protected by female military engineer' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'protected by' means one lot of letters goes inside another.
'female' becomes 'di' (short for Diana).
'military engineer' becomes 'sapper' (term for a member of the Corps of Royal Engineers).
'di'+'sapper'='disapper'
'a' going inside 'disapper' is 'DISAPPEAR'.
'from' acts as a link.
(Other definitions for disappear that I've seen before include "Pass out of sight" , "Get lost" , "walk" , "suddenly depart" , "Leave" .)