It's hard to be both a saint and a slippery customer (5)
I believe the answer is:
steel
'it's hard' is the definition.
'saint and a slippery customer' is the wordplay.
'saint' becomes 'st' (abbreviation).
'and' says to put letters next to each other.
'a slippery customer' becomes 'eel' (eel can mean a slippery person).
'st'+'eel'='STEEL'
'to be both a' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for steel that I've seen before include "Form of iron" , "Strength, determination" , "Structural metal alloy" , "Hard, strong, grey alloy" , "Knife-sharpener" .)