Getting married in it? You may eventually regret that (5)
I believe the answer is:
haste
'getting married in it? you may eventually regret that' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
This is all the clue.
Can you help me to learn more?
(Other definitions for haste that I've seen before include "Overly eager speed" , "Urgent hurry" , "impetuosity" , "Promptitude" , "Possible cause of careless mistakes?" .)