Fish starts to swim terrifically with impulse to get on (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'starts to swim terrifically with impulse to get on' is the wordplay.
'starts to' indicates taking the first letters.
'with' says to put letters next to each other.
'impulse' becomes 'urge' ('urge' can be a synonym of 'impulse'**).
'to get' means one lot of letters go next to another.
The initial letters of 'swim terrifically' is 'st'.
'st'+'urge'+'on'='STURGEON'
(Other definitions for sturgeon that I've seen before include "Large fish, valued for its roe" , "producer of caviar" , "producer of expensive eggs" , "Main resident" , "Our gents caught a large fish" .)