Yelling “Be quiet” before trip (8)
I believe the answer is:
shouting
'yelling' is the definition.
('shouting' can be a synonym of 'yelling')
'quiet before trip' is the wordplay.
'quiet' becomes 'sh'.
'before' means one lot of letters go next to another.
'trip' becomes 'outing' (I've seen this before**).
'sh'+'outing'='SHOUTING'
'be' acts as a link.
(Other definitions for shouting that I've seen before include "Talking very loudly" , "Talked very loudly" .)