Test area in former pit (7)
I believe the answer is:
examine
'test' is the definition.
(I know that test can be written as examine)
'area in former pit' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'in' means one lot of letters goes inside another.
'former' becomes 'ex' (prefix meaning former or onetime).
'pit' becomes 'mine' (mining pit).
'ex'+'mine'='exmine'
'a' going inside 'exmine' is 'EXAMINE'.
(Other definitions for examine that I've seen before include "Closely study" , "Formally question (law)" , "research" , "Test, probe" , "Check" .)