Count, maybe low, swollen by one (5)
I believe the answer is:
basie
'count maybe' is the definition.
(musician Count Basie)
'low swollen by one' is the wordplay.
'low' becomes 'base' ('base' can be a synonym of 'low'**).
'swollen by' means one lot of letters goes inside another (the word's size swells after insertion).
'one' becomes 'i' (Roman numeral).
'base' enclosing 'i' is 'BASIE'.
(Other definitions for basie that I've seen before include "Band leader" , "jazz great" , "Count ___ , US jazz pianist, d. 1984" , "Jazz musician" , "Bandleader Count" .)