Artist having lamb to start with after nil by mouth (4)
I believe the answer is:
oral
'by mouth' is the definition.
(I've seen this before)
'artist having lamb to start with after nil' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'having' says to put letters next to each other.
'to start with' says to take the initial letters.
'after' means one lot of letters go next to another.
'nil' becomes 'O' (O resembles 0).
The initial letter of 'lamb' is 'l'.
'ra' next to 'l' is 'ral'.
'ral' after 'o' is 'ORAL'.
(Other definitions for oral that I've seen before include "Relating to speech" , "Type of examination" , "Verbal" , "Said - form of test" , "Relating to the mouth - exam" .)