He composed letter after a bit of work (4)
I believe the answer is:
berg
'he composed' is the definition.
(I've seen this before)
'letter after a bit of work' is the wordplay.
'letter' becomes 'b'.
'after' means one lot of letters go next to another.
'a bit of work' becomes 'erg' (unit of work in physics).
'b'+'erg'='BERG'
(Other definitions for berg that I've seen before include "Austrian composer of 12-tone music, d. 1935" , "African mount" , "Shipping hazard; composer" , "One who scored" , "Lump of ice" .)