Politician having chat outside Parliament is warmer? (10,3)
I believe the answer is:
greenhouse gas
'is warmer?' is the definition.
(factor in global warming, i.e. it 'warms')
'politician having chat outside parliament' is the wordplay.
'politician' becomes 'green' (eg member of the Green Party).
'having' means one lot of letters go next to another.
'chat' becomes 'gas' (informal term meaning to chat).
'outside' means one lot of letters goes inside another (some letters go outside others).
'parliament' becomes 'house' (the House of Commons).
'green'+'gas'='greengas'
'greengas' going around 'house' is 'GREENHOUSE GAS'.
(Other definitions for greenhouse gas that I've seen before include "Carbon dioxide is one" , "Carbon dioxide, for example" , "will affect the climate" , "Carbon dioxide, say" , "Contributor to global warming" .)