In cracked ice, keep supple (7)
I believe the answer is:
elastic
'supple' is the definition.
(I've seen this in another clue)
'in cracked ice keep' is the wordplay.
'in' is an insertion indicator.
'cracked' indicates anagramming the letters.
'keep' becomes 'last' (I've seen this in another clue).
'ice' is an anagram of 'eic'.
'eic' enclosing 'last' is 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Stretch material" , "Flexible, adaptable" , "Cats lie (anag.)" , "Stuff that stretches to Castile" , "Castile (anag)" .)