Join American in Florence on vacation (4)
I believe the answer is:
fuse
'join' is the definition.
(I know that join can be written as fuse)
'american in florence on vacation' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'in' indicates putting letters inside.
'on vacation' says to hollow out the word (remove centre letters).
'florence' with its middle taken out is 'fe'.
'us' placed within 'fe' is 'FUSE'.
(Other definitions for fuse that I've seen before include "Safeguard in electrical circuit" , "Electrical fitting" , "Weld together through heat" , "circuit-breaker" , "Integrate" .)