Expenses break down during trips (9)
I believe the answer is:
outgoings
'expenses' is the definition.
(I've seen this before)
'break down during trips' is the wordplay.
'break down' becomes 'go' ('go' can be a synonym of 'break down').
'during' means one lot of letters goes inside another.
'trips' becomes 'outings'.
'go' going within 'outings' is 'OUTGOINGS'.
(Other definitions for outgoings that I've seen before include "expeditions" , "Regular expenditure" , "Expenses" .)