Huge parts shine, which is not actually striking (2-4)
I believe the answer is:
go-slow
'not actually striking' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'huge parts shine' is the wordplay.
'huge' becomes 'os' (abbreviation for outsize).
'parts' indicates putting letters inside (in the sense of cuts or separates).
'shine' becomes 'glow' (glow is a kind of shine).
'os' put within 'glow' is 'GO-SLOW'.
'which is' is the link.
Can you help me to learn more?
(Other definitions for go-slow that I've seen before include "Type of industrial action" , "Work-to-rule" , "Form of industrial action" .)