Come up to a very quiet fish (8)
I believe the answer is:
approach
'come up to' is the definition.
(approaching is a kind of coming up to)
'a very quiet fish' is the wordplay.
'very quiet' becomes 'pp' (musical abbreviation for pianissimo).
'fish' becomes 'roach' (roach is a kind of fish).
'a'+'pp'+'roach'='APPROACH'
(Other definitions for approach that I've seen before include "driveway" , "Get closer to" , "Advance towards" , "Avenue" , "Resemble" .)