It can be cool, half an hour in the shade (6)
I believe the answer is:
colour
'shade' is the definition.
(I've seen this before)
'it can be cool half an hour in' is the wordplay.
'it' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'can be' indicates an anagram.
'half an' indicates taking half.
'hour' becomes 'hr' (abbreviation).
'in' is an insertion indicator.
'hr' cut in half is 'r'.
'cool' anagrammed gives 'colo'.
'colo'+'r'='color'
'u' going inside 'color' is 'COLOUR'.
'the' is the link.
Can you help me to learn more?
(Other definitions for colour that I've seen before include "Quality that results from light reflection" , "Go red" , "Influence" , "Pigment, paint or dye" , "Complexion - pretext" .)