Having overindulged fast (5)
I believe the answer is:
tight
'fast' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tight that I've seen before include "Difficult and problematic" , "It's not enough" , "Awkward" , "Drunk, firm, concise" , "Mean; firmly fixed" .)