Sit in with youngster at the end of lunchtime (8)
I believe the answer is:
incubate
'sit' is the definition.
(thesaurus)
'in with youngster at the end of lunchtime' is the wordplay.
'with' is a charade indicator (letters next to each other).
'youngster' becomes 'cub' (young bear).
'the end of' suggests the final letters.
The last letter of 'lunchtime' is 'e'.
'in'+'cub'+'at'+'e'='INCUBATE'
(Other definitions for incubate that I've seen before include "Brood" , "Develop under favourable conditions, bacteria say" , "Develop slowly" , "Hatch eggs by warmth, artificial or natural" , "Keep (eggs) warm" .)