Flexible prison term? Unknown (8)
I believe the answer is:
stretchy
'flexible' is the definition.
(synonyms)
'prison term? unknown' is the wordplay.
'prison term?' becomes 'stretch' (I've seen this before).
'unknown' becomes 'y' (algebra).
'stretch'+'y'='STRETCHY'
(Other definitions for stretchy that I've seen before include "Flexible" , "Elastic" , "Of variable length, perhaps" .)