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'.
'and' means one lot of letters go next to another.
'a slippery customer' becomes 'eel' (I've seen this in another clue).
'st'+'eel'='STEEL'
'to be both a' is the link.
I am not very happy about this link. Some or all of it may be part of 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" .)