Though dishonest, he'll give £1 with a warm heart (4)
I believe the answer is:
liar
'though dishonest he'll' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'ps1 with a warm heart' is the wordplay.
'psl' becomes 'li' (I am not sure about this - if you are sure you should believe this answer much more).
'with' says to put letters next to each other.
'heart' means to look at the middle letters.
The middle of 'warm' is 'ar'.
'li'+'ar'='LIAR'
'give' acts as a link.
Can you help me to learn more?
(Other definitions for liar that I've seen before include "not straight [one]" , "One supplying pork pies" , "Fibber" , "Perjurer" , "One whose pants are on fire?" .)