Tester of prison work finally happy (6)
I believe the answer is:
canopy
'tester' is the definition.
(I've seen this before)
'prison work finally happy' is the wordplay.
'prison' becomes 'can' (the can is American slang for prison).
'work' becomes 'op' (abbreviation for opus).
'finally' suggests the final letters.
The last letter of 'happy' is 'y'.
'can'+'op'+'y'='CANOPY'
'of' is the link.
(Other definitions for canopy that I've seen before include "Roof-like shelter" , "Uppermost branches of forest trees" , "Covering against sun or rain" , "Highest level of foliage in a forest" , "Cloth covering" .)