Tetchy friend of Mole's (5)
I believe the answer is:
ratty
'tetchy' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "Fictional mole's chum" , "character on the river bank" , "One messing about in a boat" , "Characteristic of pest" , "Irritable (informal)" .)