In the main, it's marginal (3)
I believe the answer is:
hem
'in the main it's marginal' is the definition.
I can't judge whether this defines the answer.
'in the main it's marginal' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HEM' can be found hidden inside 'the main marginal'.
Can you help me to learn more?
(Other definitions for hem that I've seen before include "It draws attention to" , "Sewn border" , "A stitched border of cloth" , "Double-backed cloth border" , "Bottom of a garment" .)