Not so many sheep kept by father (5)
I believe the answer is:
fewer
'not so many' is the definition.
('few' can be the opposite of 'many' so perhaps 'fewer' is reasonable for 'not so many')
'sheep kept by father' is the wordplay.
'sheep' becomes 'ewe' (ewe is a kind of sheep).
'kept by' means one lot of letters goes inside another.
'father' becomes 'fr'.
'ewe' inserted into 'fr' is 'FEWER'.
(Other definitions for fewer that I've seen before include "Reduced" , "Less in number" , "smaller number of people" , "in smaller numbers" , "Not so many" .)