Engaged finally, I have to take the plunge! (4)
I believe the answer is:
dive
'take the plunge' is the definition.
'engaged finally i have' is the wordplay.
'finally' says to take the final letters.
'i have' becomes 'ive' (**).
The final letter of 'engaged' is 'd'.
'd'+'ive'='DIVE'
'to' is the link.
(Other definitions for dive that I've seen before include "Dip" , "Plunge; low bar" , "Disreputable nightclub" , "Low establishment" , "sleazy bar" .)