Burn recusant's head in scalding vapour (6)
I believe the answer is:
stream
'burn' is the definition.
(burn is a Scottish dialect word for a stream)
'recusant's head in scalding vapour' is the wordplay.
'head' suggests taking the first letters.
'in' indicates putting letters inside.
'scalding vapour' becomes 'steam' (hot vapour from boiling water).
The first letter of 'recusants' is 'r'.
'r' placed into 'steam' is 'STREAM'.
(Other definitions for stream that I've seen before include "Rill, brook" , "Constant flow of liquid" , "Natural body of running water" , "Small narrow flow" , "group of schoolchildren" .)