Put on extended promotion (3)
I believe the answer is:
add
'put on' is the definition.
(I've seen this in another clue)
'extended promotion' is the wordplay.
I cannot quite see how this works, but
'promotion' could be 'ad' (short for advert) and 'ad' is found within the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for add that I've seen before include "Put together (numbers)" , "Say as a further remark" , "Sum up; say further" , "Give more" , "problem for hyperactive children" .)