Cast is turning fat – a hindrance when on the road (7)
I believe the answer is:
bollard
'a hindrance when on the road' is the definition.
I can't judge whether this defines the answer.
'cast is turning fat' is the wordplay.
'cast' becomes 'lob'.
'is turning' says the letters should be written in reverse.
'fat' becomes 'lard' (lard is a kind of fat).
'lob' backwards is 'bol'.
'bol'+'lard'='BOLLARD'
Can you help me to learn more?
(Other definitions for bollard that I've seen before include "Thick post for mooring" , "Short post barring traffic" , "Post on road or quay" , "Strong post" , "Roadside post" .)