Little bit of Africa to investigate, travelling north (4)
I believe the answer is:
iota
'little bit' is the definition.
(thesaurus)
'of africa to investigate travelling north' is the wordplay.
'of' says the answer is hidden in the clue.
'travelling north' shows that the letters should be reversed in order (in a down clue, letters go up).
'atoi' is hidden in the letters of 'africa to investigate'.
'atoi' in reverse letter order is 'IOTA'.
(Other definitions for iota that I've seen before include "Ninth letter of Greek alphabet, small amount" , "Just a jot, like Greek letter" , "(Not one) little bit" , "Ninth Greek character" , "Grain" .)