Swimmer's initially excited by terms of the deal (3)
I believe the answer is:
eel
'swimmer's' is the definition.
(eel is a type of fish)
'initially excited by terms of the deal' is the wordplay.
'initially' suggests taking the first letters.
'by' means one lot of letters go next to another.
'terms of' indicates one should take the final letters (term can mean the end of something).
The first letter of 'excited' is 'e'.
The final letters of 'the deal' are 'el'.
'e'+'el'='EEL'
(Other definitions for eel that I've seen before include "Snakelike fish" , "Snaky fish" , "Creature like conger or moray" , "Slippery type" , "User of electric organ" .)