Crack appears in glacier, if thawing (4)
I believe the answer is:
rift
'crack' is the definition.
(rift is a kind of crack)
'in glacier if thawing' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RIFT' is hidden in the letters of 'glacier if thawing'.
'appears' is the link.
(Other definitions for rift that I've seen before include "Breach; crack" , "gap" , "Fissure in something" , "A disagreement or an earth cleft" , "opening" .)