Primates have been active, primarily in hats (10)

'primates' is the definition.
(bushbaby is a kind of primate)

'have been active primarily in hats' is the wordplay.
'primarily' says to take the initial letters.
'in' is an insertion indicator.
'hats' becomes 'busbies' (busby is a kind of hat).
The initial letters of 'have been active' is 'hba'.
'hba' inserted inside 'busbies' is 'BUSHBABIES'.