Supplies more promotional material, doubling content (4)
I believe the answer is:
adds
'supplies' is the definition.
('add' can be a synonym of 'supply')
'promotional material doubling content' is the wordplay.
'promotional material' becomes 'ads' (I've seen this in another clue).
'doubling' becomes 'd' (this could be a standard abbreviation of which I'm unaware).
'content' means one lot of letters goes inside another (some letters act as content for another word).
'ads' going around 'd' is 'ADDS'.
'more' is the link.
Can you help me to learn more?
(Other definitions for adds that I've seen before include "Puts (numbers) together" , "Reckons" , "Sums, totals" , "Counts" , "Aggregates or makes further remark" .)