Aerial I'd put back on a post (6)
I believe the answer is:
dipole
'aerial' is the definition.
(I know that dipole is a type of aerial)
'i'd put back on a post' is the wordplay.
'put back' says the letters should be written backwards.
'on' means one lot of letters go next to another.
'a post' becomes 'pole' (I've seen this in other clues).
'id' backwards is 'di'.
'di'+'pole'='DIPOLE'
(Other definitions for dipole that I've seen before include "I loped between two equal and opposite electric charges" , "Directional TV or radio aerial" , "Type of aerial" .)