Warm when placed, say, upside down (6)
I believe the answer is:
genial
'warm' is the definition.
(I've seen this in another clue)
'placed say upside down' is the wordplay.
'placed' becomes 'lain' (I've seen this in another clue).
'say' becomes 'eg' (both can mean for example).
'upside down' says the letters should be written backwards.
'lain'+'eg'='laineg'
'laineg' backwards is 'GENIAL'.
'when' is the link.
(Other definitions for genial that I've seen before include "Amiable - linage (anag)" , "'Cheerful, friendly, sociable (6)'" , "Mild and pleasant" , "Cheerful and easy-going" , "'Affable, warm and friendly (6)'" .)