Join in swimming finally in lake (5)
I believe the answer is:
merge
'join in' is the definition.
(thesaurus)
'swimming finally in lake' is the wordplay.
'finally' says to take the final letters.
'in' is an insertion indicator.
'lake' becomes 'mere' (mere is a kind of lake).
The last letter of 'swimming' is 'g'.
'g' placed into 'mere' is 'MERGE'.
(Other definitions for merge that I've seen before include "Amalgamate" , "Mix" , "Combine into one" , "Come together as one" , "Join, blend" .)