Feature of koala gestation periods (4)
I believe the answer is:
ages
'periods' is the definition.
(age is a kind of period)
'feature of koala gestation' is the wordplay.
'feature of' indicates the answer is hidden within the clue.
'AGES' is hidden in the letters of 'koala gestation'.
(Other definitions for ages that I've seen before include "(What seems) a very long time" , "Yonks" , "Historic periods or prolonged time" , "Donkey's years" , "... of Man, according to WB Yeats" .)