Half of pies and more pasty knocked back - that's degenerate (7)
I believe the answer is:
relapse
'that's degenerate' is the definition.
(to relapse is to degenerate or regress)
'half of pies and more pasty knocked back' is the wordplay.
'half of' suggests halving the letters.
'and' says to put letters next to each other.
'more pasty' becomes 'paler' ('pale' can be a synonym of 'pasty').
'knocked back' says the letters should be written in reverse.
'pies' halved is 'es'.
'es'+'paler'='espaler'
'espaler' in reverse letter order is 'RELAPSE'.
(Other definitions for relapse that I've seen before include "Deteriorate in health" , "setback in health terms" , "Slide back into former condition as of illness (7)" , "Fall back into former condition, being pleaser" , "Fall back into old ways" .)