Father also included small bears (6)
I believe the answer is:
pandas
'bears' is the definition.
(pandas are bears)
'father also included small' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'also' becomes 'and' (both can mean in addition).
'included' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pa' going around 'and' is 'panda'.
'panda'+'s'='PANDAS'
(Other definitions for pandas that I've seen before include "Chinese bear-like mammals" , "endangered animals" , "wild animals from Asia" , "Black-and-white mammals of bamboo forests" , "Black and white Chinese bears" .)