Pine toilets: they’re warm in winter (4,5)
I believe the answer is:
long johns
'they're warm in winter' is the definition.
(warm trousers which might be worn in winter)
'pine toilets' is the wordplay.
'pine' becomes 'long' (synonyms).
'toilets' becomes 'johns' ('john' can be a synonym of 'toilet').
'long'+'johns'='LONG JOHNS'
(Other definitions for long johns that I've seen before include "Old habits" , "Pants with legs" , "warm clothing" , "Robert Louis's hero" , "Underpants with legs" .)