Rising every so often, man took in a very small amount (4)
I believe the answer is:
iota
'a very small amount' is the definition.
(I've seen this in another clue)
'rising every so often man took in' is the wordplay.
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'every so often' indicates alternate letters.
'man'+'took'+'in'='mantookin'
The alternate letters of 'mantookin' are 'atoi'.
'atoi' reversed gives 'IOTA'.
(Other definitions for iota that I've seen before include "Just a jot, from Greek alphabet" , "'Very small amount, jot (4)'" , "Just a jot, like Greek letter" , "Hint" , "Ninth Greek character" .)