Was eating female monarch's biscuits (6)
I believe the answer is:
wafers
'biscuits' is the definition.
(wafer is a kind of biscuit)
'was eating female monarch's' is the wordplay.
'eating' means one lot of letters goes inside another.
'female' becomes 'f' (abbreviation).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'f'+'er'='fer'
'was' enclosing 'fer' is 'WAFERS'.
(Other definitions for wafers that I've seen before include "Thin biscuits, may hold ice cream" , "Small, thin, crisp biscuits" , "Very thin biscuits, may hold ice-cream" , "Very thin buscuits." , "Very thin plain biscuits" .)