Zest shown by someone disappointing PM once (5,4)
I believe the answer is:
lemon peel
'zest' is the definition.
(I've seen this before)
'someone disappointing pm once' is the wordplay.
I cannot really understand how this works, but
'pm' could be 'peel' (I've seen this before) and 'peel' is found in the answer.
The remaining letters 'lemon' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'shown by' acts as a link.
Can you help me to learn more?
(Other definitions for lemon peel that I've seen before include "twist" , "Source of zest" .)