More insubstantial theory raised in leaflet (8)
I believe the answer is:
flimsier
'more insubstantial' is the definition.
Both the definition and answer are comparative adjectives. Maybe you can see a link between them that I can't see?
'theory raised in leaflet' is the wordplay.
'theory' becomes 'ism' (I've seen this in another clue).
'raised' says the letters should be written in reverse (going up in a down clue).
'in' indicates putting letters inside.
'leaflet' becomes 'flier' (I've seen this in another clue).
'ism' reversed gives 'msi'.
'msi' placed into 'flier' is 'FLIMSIER'.
Can you help me to learn more?
(Another definition for flimsier that I've seen is " More fragile, slighter".)