Stretch old pot, on the day (6)
I believe the answer is:
expand
'stretch' is the definition.
(thesaurus)
'old pot on the day' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'pot' becomes 'pan' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'the day' becomes 'd' (abbreviation for day).
'ex'+'pan'+'d'='EXPAND'
(Other definitions for expand that I've seen before include "Become larger or more extensive" , "Fill out" , "Branch out" , "Make more extensive" , "Swell" .)