Tester needs one to probe ape (6)
I believe the answer is:
canopy
'tester' is the definition.
(thesaurus)
'one to probe ape' is the wordplay.
'one' becomes 'an' (I've seen this in other clues).
'to probe' indicates putting letters inside.
'ape' becomes 'copy' ('copy' can be a synonym of 'ape').
'an' put inside 'copy' is 'CANOPY'.
'needs' acts as a link.
(Other definitions for canopy that I've seen before include "Upper layer of a forest" , "Roof-like awning" , "Covering serving as shelter" , "Hanging cover over bed, shopfront, etc" , "Cockpit cover" .)