Ink discharging both ends -- a pen's dreadful (5)
I believe the answer is:
nasty
'dreadful' is the definition.
(thesaurus)
'ink discharging both ends a pen's' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'pen' could be 'sty' (both can mean an animal enclosure) and 'sty' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nasty that I've seen before include "Disgusting" , "Not nice" , "Highly unpleasant" , "Vile" , "rank" .)