Recess's contents: yellow or colourless fluid (5)
I believe the answer is:
ichor
'colourless fluid' is the definition.
'ichor' can be an answer for 'fluid' (ichor is a kind of fluid). I'm not sure about the 'colourless' bit.
'recess's contents yellow' is the wordplay.
'recess's contents' becomes 'ich' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'yellow' becomes 'or' (heraldic term).
'ich'+'or'='ICHOR'
'or' is the link.
Can you help me to learn more?
(Other definitions for ichor that I've seen before include "flows divinely" , "divine claret?" , "Ethereal juice" , "Superior blood" , "Blood-fluid of the gods" .)