Small firm finally went national (4)
I believe the answer is:
scot
'national' is the definition.
(someone from Scotland)
'small firm finally went' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'firm' becomes 'co' (a firm is a company).
'finally' says to take the final letters.
The last letter of 'went' is 't'.
's'+'co'+'t'='SCOT'
(Other definitions for scot that I've seen before include "Someone from Dundee?" , "Glaswegian perhaps" , "Northerner; old tax" , "Eg, a Glaswegian" , "Native of Glasgow, say" .)