Burn plan to release tension (4)
I believe the answer is:
char
'burn' is the definition.
(charring is a kind of burning)
'plan to release tension' is the wordplay.
'plan' becomes 'chart' (chart can mean a plan or diagram).
'to release' indicates named letters should be taken away (letters are released or freed).
'tension' becomes 't' (abbreviation for surface tension).
'chart' with 't' taken away is 'CHAR'.
(Other definitions for char that I've seen before include "'Mrs Mopp'" , "Kind of small trout - burn it black" , "Fish < tea" , "Burn slightly" , "Drink" .)