He fibs about a mode of transport (4)
I believe the answer is:
liar
'he fibs' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'about a mode of transport' is the wordplay.
'about' says the letters should be written in reverse.
'a mode of transport' becomes 'rail' (I've seen this before).
'rail' written backwards gives 'LIAR'.
Can you help me to learn more?
(Other definitions for liar that I've seen before include "One telling untruths" , "Fibber" , "man of fiction?" , "one trying to sell pork pies" , "one telling a story" .)