A shade cool, perhaps, for half an hour (6)
I believe the answer is:
colour
'a shade' is the definition.
(I've seen this before)
'cool perhaps for half an hour' is the wordplay.
'perhaps' indicates anagramming the letters.
'for' means one lot of letters go next to another.
'half an hour' becomes 'ur'.
'cool' is an anagram of 'colo'.
'colo'+'ur'='COLOUR'
(Other definitions for colour that I've seen before include "Hue, shade" , "black?" , "'Tinge, hue (6)'" , "Quality that results from light reflection" , "Red or green, say" .)