Crony that’s laid up, eating fish kedgeree at first (8)
I believe the answer is:
'crony' is the definition.
('sidekick' can be a synonym of 'crony')
'laid up eating fish kedgeree at first' is the wordplay.
'laid up' becomes 'sick' ('sick' can be similar in meaning to 'laid up').
'eating' indicates putting letters inside.
'fish' becomes 'ide' (I've seen this before**).
'at first' suggests taking the first letters.
The first letter of 'kedgeree' is 'k'.
'sick' enclosing 'idek' is 'SIDEKICK'.
'that's' acts as a link.
(Other definitions for sidekick that I've seen before include "mate" , "Assistant to a more dominant person (informal)" , "Buddy, constant companion" , "Person's assistant" , "Acolyte" .)