It's foolish to be foxy about just one pound (5)
I believe the answer is:
silly
'it's foolish' is the definition.
(similar in meaning)
'foxy about just one pound' is the wordplay.
'foxy' becomes 'sly' (synonyms).
'about' indicates putting letters inside.
'just one' becomes 'i' (Roman numeral. I am not sure about the 'just' bit.).
'pound' becomes 'l'.
'i'+'l'='il'
'sly' going around 'il' is 'SILLY'.
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for silly that I've seen before include "Trivial, frivolous" , "Absurd" , "Ludicrous, foolish" , "Daft" , "Ridiculous, frivolous" .)