Lovers' tiff ending, somewhat tense (7)
I believe the answer is:
stiffen
'tense' is the definition.
(thesaurus)
'lovers' tiff ending somewhat' is the wordplay.
'somewhat' indicates a hidden word.
'STIFFEN' is hidden within 'lovers tiff ending'.
(Other definitions for stiffen that I've seen before include "Steel" , "Grow harder" , "more severe" , "Make unbending" , "Reinforce" .)