Dad's visibly angry when trimmed! (5)
I believe the answer is:
pared
'trimmed' is the definition.
('pare' can be a synonym of 'trim')
'dad's visibly angry' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'visibly angry' becomes 'red' (red in the face).
'pa'+'red'='PARED'
'when' is the link.
(Other definitions for pared that I've seen before include "Of a fruit, peeled" , "Trimmed like pencil." , "Peeled the outer surface" , "Trimmed - peeled" , "Did prune" .)