Fast eating that is initially not sparing (7)
I believe the answer is:
lenient
'sparing' is the definition.
(thesaurus)
'fast eating that is initially not' is the wordplay.
'fast' becomes 'lent' (I've seen this before).
'eating' is an insertion indicator.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'initially' says to take the initial letters.
The initial letter of 'not' is 'n'.
'ie'+'n'='ien'
'lent' enclosing 'ien' is 'LENIENT'.
(Other definitions for lenient that I've seen before include "Forgiving, soft" , "Light" , "Mild and tolerant" , "Indulgent, lax" , "Clement" .)