Contract, massive thing by the sound of it (7)
I believe the answer is:
tighten
'contract' is the definition.
(I've seen this before)
'massive thing by the sound of it' is the wordplay.
'massive thing' becomes 'titan'.
'by the sound of it' shows a homophone (sound like).
'titan' is a homophone of 'TIGHTEN'.
(Other definitions for tighten that I've seen before include "Make harder to undo" , "Stiffen" , "Constrict" , "Make more restrictive" , "becoming tense" .)