Hardly worth mentioning (7)
(Other definitions for trivial that I've seen before include "Meaningless" , "Rival it, being unimportant" , "Of little consequence" , "Very easy" , "Worthless" .)
'hardly worth mentioning' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nominal that I've seen before include "Ironical understatement (gram mar.)" , "Trivial" , "In name only or of token value (7)" , "In name or form only" , "Minor" .)