Shackles press savagely at first (5)
I believe the answer is:
irons
'shackles' is the definition.
(iron is a kind of shackle)
'press savagely at first' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'at first' indicates taking the first letters.
The first letter of 'savagely' is 's'.
'iron'+'s'='IRONS'
(Other definitions for irons that I've seen before include "instruments of restraint" , "Don't have too many in the fire at the same time" , "Smooths the golfclubs?" , "Imposers of restrictions" , "Golf-clubs with metal heads" .)