Butt in middle of ashtray held by old boy with no manners (7)
I believe the answer is:
obtrude
'butt in' is the definition.
(thesaurus)
'middle of ashtray held by old boy with no manners' is the wordplay.
'middle of' means to look at the middle letters.
'held by' indicates putting letters inside.
'old boy' becomes 'ob' (abbreviation).
'with no manners' becomes 'rude'.
The central letter of 'ashtray' is 't'.
'ob'+'rude'='obrude'
't' going within 'obrude' is 'OBTRUDE'.
(Other definitions for obtrude that I've seen before include "Become unwelcomely prominent" , "Thrust oneself forward" , "Make unpleasantly noticeable" , "Be offensively prominent" , "Redoubt (anag) - become noticeable in an unwelcome way" .)