It's very hot when it's clear on the hill (6)
I believe the answer is:
torrid
'it's very hot' is the definition.
(I've seen this before)
'it's clear on the hill' is the wordplay.
'it's clear' becomes 'rid' ('rid' can be a synonym of 'clear').
'on' means one lot of letters go next to another.
'the hill' becomes 'tor' (tor is a kind of hill).
'rid' put after 'tor' is 'TORRID'.
'when' is the link.
(Other definitions for torrid that I've seen before include "sultry atmosphere" , "Scorching hot" , "Extremely hot" , "Bendy" , "Dried with heat or intensely passionate" .)