One nominally involved? Gosh, no, the reverse of that (6)
I believe the answer is:
eponym
'of that' is the definition.
The definition suggests a singular noun which matches the answer.
'one nominally involved? gosh no the reverse' is the wordplay.
'one nominally involved?' becomes 'ep' (I am not sure about this - if you are sure you should believe this answer much more).
'gosh' becomes 'my' (both are old-fashioned expressions of surprise).
'the reverse' is a reversal indicator.
'my'+'no'='myno'
'myno' written backwards gives 'onym'.
'ep'+'onym'='EPONYM'
Can you help me to learn more?
(Other definitions for eponym that I've seen before include "such as Black Beauty" , "handle from place?" , "someone giving a name" , "a mackintosh, for instance" , "Word or place name derived from person's name" .)