Reportedly giant is tense (7)
I believe the answer is:
tighten
'is tense' is the definition.
(I've seen this before)
'reportedly giant' is the wordplay.
'reportedly' shows a homophone (sound like).
'giant' becomes 'titan' ('titan' can be a synonym of 'giant').
'titan' is a homophone of 'TIGHTEN'.
(Other definitions for tighten that I've seen before include "Make tauter" , "contract" , "get tense" , "Make (rope) less slack" , "becoming tense" .)