Get closer to one extremely plump fish (8)
I believe the answer is:
approach
'get closer to' is the definition.
(I've seen this before)
'one extremely plump fish' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'extremely' says to hollow out the word (remove centre letters) (letters on the extremes of the word are used).
'fish' becomes 'roach' (roach is a type of fish).
'plump' with its middle taken out is 'pp'.
'a'+'pp'+'roach'='APPROACH'
(Other definitions for approach that I've seen before include "Go towards" , "Drae near" , "driveway" , "What happens before landing" , "attitude" .)