I return to a smidgeon (4)
I believe the answer is:
iota
'smidgeon' is the definition.
(I've seen this in another clue)
'i return to a' is the wordplay.
'return' says the letters should be written backwards.
'to' back-to-front is 'ot'.
'i'+'ot'+'a'='IOTA'
(Other definitions for iota that I've seen before include "Ninth Greek character" , "Ninthe letter of the Greek alphabet" , "A little piece" , "Extremely small amount" , "Greek letter; tiny amount" .)