Prison term or a grammatical unit (8)
I believe the answer is:
sentence
This clue is a double definition.
'prison term' is the first definition.
(synonyms)
'a grammatical unit' is the second definition.
(I've seen this in another clue)
(Other definitions for sentence that I've seen before include "Judgement passed on criminal" , "Judicial penalty" , "pronouncement from the judge" , "That which ends with a full stop" , "Group of words with verb" .)