Secure giant, say (7)
I believe the answer is:
tighten
'secure' is the definition.
(thesaurus)
'giant say' is the wordplay.
'giant' becomes 'titan' (titans are giant mythological creatures).
'say' indicates a 'sounds like' (homophone) clue.
'titan' sounds like 'TIGHTEN'.
(Other definitions for tighten that I've seen before include "increase tension" , "Make more restrictive" , "Turn, a screw say" , "Fasten more firmly" , "Constrict" .)