Colour of a courtyard, mainly (4)
I believe the answer is:
aqua
'colour' is the definition.
(aqua is a kind of colour)
'a courtyard mainly' is the wordplay.
'courtyard' becomes 'quad' (short for quadrangle).
'mainly' means to remove the last letter.
'quad' with its final letter taken away is 'qua'.
'a'+'qua'='AQUA'
'of' is the link.
(Other definitions for aqua that I've seen before include "Tint of blue" , "Light bluish-green -- water" , "-- fortis, 'strong water'" , "Hue" , "which supports boats?" .)