30 or 21 say, for instance put in ground outside (7)
I believe the answer is:
integer
'30 or 21 say' is the definition.
'for instance put in ground outside' is the wordplay.
'for instance' becomes 'eg' ('e.g.' can be a synonym of 'for instance').
'put in ground' becomes 'inter' (I've seen this in other clues).
'outside' is an insertion indicator.
'eg' put within 'inter' is 'INTEGER'.
(Other definitions for integer that I've seen before include "A whole number, not a fraction" , "Whole unit" , "one or two, perhaps" , "Treeing (anag.)" , "One for example" .)