Genuine ceasefire, though not constant (4)
I believe the answer is:
true
'genuine' is the definition.
(synonyms)
'ceasefire though not constant' is the wordplay.
'ceasefire' becomes 'truce' (I've seen this before).
'though not' indicates named letters should be taken away.
'constant' becomes 'c' (mathematical abbreviation).
'truce' with 'c' removed is 'TRUE'.
(Other definitions for true that I've seen before include "Honest and straight" , "Exact, factual" , "Veritable" , "A false alternative?" , "Real, actual" .)