Prison term or grammatical unit (8)
I believe the answer is:
sentence
This clue is a double definition.
'prison term' is the first definition.
(synonyms)
'grammatical unit' is the second definition.
(I've seen this before)
(Other definitions for sentence that I've seen before include "Part of a message" , "Words - said by judge" , "That which ends with a full stop" , "in so many words" , "Court decision on a grammatical structure" .)