Gondoliers, say, discovered a source of luxury food (7)
I believe the answer is:
oatmeal
'food' is the definition.
(oatmeal is a kind of food)
'gondoliers say discovered a source of luxury' is the wordplay.
'gondoliers say' becomes 'boatmen' (gondolier is a kind of boatman).
'discovered' says to take the centre (dis-covered - outer letters removed).
'a source of luxury' becomes 'al' (I can't justify this - if you can you should give a lot more credence to this answer).
The central letters of 'boatmen' are 'oatme'.
'oatme'+'al'='OATMEAL'
Can you help me to learn more?
(Other definitions for oatmeal that I've seen before include "Colour - to a male (anag)" , "It's boiled for porridge" , "Ground cereal" , "AA Motel (anag.)" , "Ground grain used to make porridge" .)