Poor saint behind tree shat trousers (4-8)
I believe the answer is:
cash-strapped
'poor' is the definition.
'saint behind tree shat trousers' is the wordplay.
I cannot really understand how this works, but
'saint' could be 'st' (abbreviation) and 'st' is found within the answer.
'tree' could be 'ash' (ash is a kind of tree) and 'ash' is found in the answer.
The remaining letters 'crapped' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cash-strapped that I've seen before include "Short of money (informal)" , "Extremely short of money" .)