Composer mixed up lines and space (7)
I believe the answer is:
nielsen
'composer' is the definition.
(I've seen this before)
'mixed up lines and space' is the wordplay.
'mixed up' is an anagram indicator.
'and' is a charade indicator (letters next to each other).
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'lines' with letters rearranged gives 'niels'.
'niels'+'en'='NIELSEN'
(Other definitions for nielsen that I've seen before include "Danish composer, Carl (1865-1931)" , "Danish symphonist" , "Carl -, Dan. composer" , "Musician" , "Carl -, Danish cmpsr." .)