Mean chaps heading off with dog (6)
I believe the answer is:
entail
'mean' is the definition.
('entail' can be a synonym of 'mean')
'chaps heading off with dog' is the wordplay.
'chaps' becomes 'men' (I've seen this before).
'heading off' means to remove the first letter.
'with' means one lot of letters go next to another.
'dog' becomes 'tail' ('tail' can be a synonym of 'dog'**).
'men' with its initial letter taken off is 'en'.
'en'+'tail'='ENTAIL'
(Other definitions for entail that I've seen before include "Bring on, involve" , "Involve (something) as an inevitable consequence" , "Cause" , "Involve or require" , "legal restriction" .)