Strap ideally holds fast (5)
I believe the answer is:
rapid
'fast' is the definition.
(similar in meaning)
'strap ideally holds' is the wordplay.
'holds' indicates a hidden word.
'RAPID' is hidden in the letters of 'strap ideally'.
(Other definitions for rapid that I've seen before include "Fast, quick" , "moving fast" , "Happening quickly" , "Speedy, quick" , "Moving at speed" .)