For a start, a dog may be in one! (4)
I believe the answer is:
trap
'be in one' is the definition.
The definition and answer can be both to do with social activities as well as being verbs in their base form.
Maybe you can see an association between them that I can't see?
'for a start a dog' is the wordplay.
'for' becomes 'to' (eg both can mean 'in the opinion of').
'a start' means to remove the last letter (I've seen 'starts' mean this).
'a dog' becomes 'rap' (I can't explain this - if you can you should believe this answer much more).
'to' with its last letter taken away is 't'.
't'+'rap'='TRAP'
'may' acts as a link.
Can you help me to learn more?
(Other definitions for trap that I've seen before include "Confine" , "Driver's obstacle" , "Cakehole (slang)" , "Lock" , "that may catch a speeding motorist" .)