Very small quantity of iron that’s taken regularly (4)
I believe the answer is:
iota
'very small quantity' is the definition.
(I've seen this in another clue)
'iron that's taken regularly' is the wordplay.
'taken regularly' indicates alternate letters (letters removed at regular intervals).
'iron'+'that'='ironthat'
The alternate letters of 'ironthat' are 'IOTA'.
'of' is the link.
(Other definitions for iota that I've seen before include "Contrivance" , "Old letter" , "Greek letter; tiny amount" , "Just a jot, like Greek letter" , "Extremely small amount" .)