They're of marginal significance (5)
I believe the answer is:
edges
'they're' is the definition.
The definition suggests an adverb but the answer is not.
'marginal significance' is the wordplay.
'marginal' becomes 'edge' (synonyms).
'significance' becomes 's' (this could be a standard abbreviation I've not previously seen).
'edge'+'s'='EDGES'
'of' is the link.
Can you help me to learn more?
(Other definitions for edges that I've seen before include "Outer limits" , "Borders, rims" , "They're farthest from the middle" , "Margins" , "Bounds" .)