Swelters with dampness up on board (5)
I believe the answer is:
stews
'swelters' is the definition.
(I've seen this in another clue)
'with dampness up on board' is the wordplay.
'with' is an insertion indicator.
'dampness' becomes 'wet' (I've seen this in another clue).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'on board' becomes 'ss' (I've seen this in other clues).
'wet' written backwards gives 'tew'.
'tew' inserted within 'ss' is 'STEWS'.
(Other definitions for stews that I've seen before include "Ragouts" , "Dishes cooked by long simmering" , "Simmers" , "Pickles" , "Cooks slowly in liquid" .)