Displeased at being a single pound in the red (5)
I believe the answer is:
riled
'displeased at' is the definition.
'riled' can be an answer for 'displeased' (riling is a kind of displeasing). I'm not certain of the 'at' bit.
'a single pound in the red' is the wordplay.
'a single' becomes 'I' (one).
'pound' becomes 'l' (the pound sign is based on an L).
'in' indicates putting letters inside.
'i'+'l'='il'
'il' put inside 'red' is 'RILED'.
'being' acts as a link.
Can you help me to learn more?
(Other definitions for riled that I've seen before include "Nettled" , "Vexed" , "Cross" , "get angry" , "Irritated, provoked" .)