Warm to loosen up (4)
I believe the answer is:
thaw
I believe this is a double definition.
'warm' is the first definition.
(thesaurus)
'loosen up' is the second definition.
(I've seen this in another clue)
'to' acts as a link.
(Other definitions for thaw that I've seen before include "What - melt like snow?" , "Morse actor; melt" , "De-ice" , "Become liquid" , "John, once regularly on TV" .)