Taking rest of Flake back in envy, perhaps the origin of gluttony (8)

'taking rest' is the definition.
(sleeping means having a rest)

'flake back in envy perhaps the origin of gluttony' is the wordplay.
'flake' becomes 'peel' (synonyms).
'back' is a reversal indicator.
'in' indicates putting letters inside.
'envy perhaps' becomes 'sin' (envy is a kind of sin).
'the origin of' suggests taking the first letters.
The initial letter of 'gluttony' is 'g'.
'peel' backwards is 'leep'.
'leep' put into 'sin' is 'sleepin'.
'sleepin'+'g'='SLEEPING'

'of' is the link.