Colossus reportedly becoming tense (7)
I believe the answer is:
tighten
'becoming tense' is the definition.
(I've seen this before)
'colossus reportedly' is the wordplay.
'colossus' becomes 'titan' (both can mean something very big).
'reportedly' indicates a 'sounds like' (homophone) clue.
'titan' is a homophone of 'TIGHTEN'.
(Other definitions for tighten that I've seen before include "contract" , "Make tenser" , "increase tension" , "Restrict" , "Make more strict" .)