In cavities, raising little birds (7)

'birds' is the definition.
(peewit is a kind of bird)

'in cavities raising little' is the wordplay.
'in' means one lot of letters goes inside another.
'cavities' becomes 'pits' ('pit' can be a synonym of 'cavity').
'raising' shows that the letters should be reversed in order.
'little' becomes 'wee' ('wee' can be similar in meaning to 'little'**).
'wee' back-to-front is 'eew'.
'pits' enclosing 'eew' is 'PEEWITS'.