Look into quarry trial (5)
I believe the answer is:
pilot
'trial' is the definition.
(I've seen this before)
'look into quarry' is the wordplay.
'look' becomes 'lo'.
'into' is an insertion indicator.
'quarry' becomes 'pit' ('pit' can be a synonym of 'quarry').
'lo' placed into 'pit' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Person in charge of aircraft" , "who shows the way?" , "One-off programme" , "Lead" , "Drive" .)