Sentence trusty ultimately finds elastic (8)
I believe the answer is:
stretchy
'elastic' is the definition.
(similar in meaning)
'sentence trusty ultimately' is the wordplay.
'sentence' becomes 'stretch' (a stretch in prison).
'ultimately' indicates one should take the final letters.
The final letter of 'trusty' is 'y'.
'stretch'+'y'='STRETCHY'
'finds' acts as a link.
(Other definitions for stretchy that I've seen before include "Of variable length, perhaps" , "Flexible" , "Elastic" .)