To be truthful, it could be nicer in the southeast (7)
I believe the answer is:
sincere
'to be truthful' is the definition.
(I've seen this in another clue)
'it could be nicer in the southeast' is the wordplay.
'it could be' is an anagram indicator (the same letters could be an anagram).
'in the' indicates putting letters inside.
'southeast' becomes 'se' (synonyms).
'nicer' anagrammed gives 'incer'.
'incer' inserted into 'se' is 'SINCERE'.
(Other definitions for sincere that I've seen before include "Not hypocritical" , "straightforward" , "Perfect" , "Genuine; deceit-free" , "Free from deceit" .)