Mountain ash up in Ballina worm-affected (5)
I believe the answer is:
rowan
'mountain ash' is the definition.
(rowan is a kind of mountain ash)
'up in ballina worm-affected' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'nawor' is hidden in the letters of 'ballina worm affected'.
'nawor' backwards is 'ROWAN'.
(Other definitions for rowan that I've seen before include "Mr Atkinson, actor" , "-- Williams, theologian" , "Tree - archbishop" , "Mountain-ash" , "Tree with orange-red berry-like fruits" .)