Donkey's brother carrying guard occasionally (5)
I believe the answer is:
burro
'donkey's' is the definition.
(burro is a kind of donkey)
'brother carrying guard occasionally' is the wordplay.
'brother' becomes 'bro'.
'carrying' means one lot of letters goes inside another.
'occasionally' means one should take alternating letters.
The alternate letters of 'guard' are 'ur'.
'bro' placed around 'ur' is 'BURRO'.
(Other definitions for burro that I've seen before include "Spanish donkey" , "Small donkey (US)" , "Small donkey used as a pack animal" , "Beast of burden" .)