Old quarry conceals a test (7)
I believe the answer is:
examine
'test' is the definition.
(I know that test can be written as examine)
'old quarry conceals a' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'quarry' becomes 'mine' (type of mine).
'conceals' indicates putting letters inside.
'ex'+'mine'='exmine'
'exmine' enclosing 'a' is 'EXAMINE'.
(Other definitions for examine that I've seen before include "Appraise" , "research" , "Inspect, study" , "Question (a witness)" , "Check" .)