Current king in Hamlet replaced (7)
I believe the answer is:
thermal
'current' is the definition.
(I've seen this before)
'king in hamlet replaced' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'in' indicates putting letters inside.
'replaced' indicates anagramming the letters.
'hamlet' with letters rearranged gives 'themal'.
'r' put inside 'themal' is 'THERMAL'.
(Other definitions for thermal that I've seen before include "Warm or hot - underwear?" , "Related to heat" , "Her malt became heat-related" , "type of baths" , "Designed to protect from cold" .)