Giant said to screw up (7)
I believe the answer is:
tighten
'screw up' is the definition.
(I've seen this before)
'giant said' is the wordplay.
'giant' becomes 'titan' (titans are giant mythological creatures).
'said' shows a homophone (sound like).
'titan' is a homophone of 'TIGHTEN'.
'to' acts as a link.
(Other definitions for tighten that I've seen before include "get tense" , "put the squeeze on" , "Give an extra turn" , "To improve focus" , "Make less loose" .)