Fit agent welcoming a swim on vacation (8)
I believe the answer is:
paroxysm
'fit' is the definition.
(I know that fit can be written as paroxysm)
'agent welcoming a swim on vacation' is the wordplay.
'agent' becomes 'proxy' (proxy is a kind of agent).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'on vacation' says to hollow out the word (remove centre letters).
'swim' with its centre removed is 'sm'.
'proxy' placed around 'a' is 'paroxy'.
'paroxy'+'sm'='PAROXYSM'
(Other definitions for paroxysm that I've seen before include "Fit - convulsion" , "Sudden uncontrollable outburst - fit" , "Uncontrollable fit (of giggles, coughing etc)" , "Bloke" , "Spasm" .)