Frozen matter encountered in Arctic exploration (3)
I believe the answer is:
ice
'frozen matter' is the definition.
(frozen water)
'encountered in arctic exploration' is the wordplay.
'encountered in' indicates a hidden word.
'ICE' is hidden within 'arctic exploration'.
(Other definitions for ice that I've seen before include "Cubes in a drink" , "Some number" , "Frozen water; kill (slang)" , "Skating surface" , "Do cake decoration" .)