Nourished iron deficiency, at the outset (3)
I believe the answer is:
fed
'nourished' is the definition.
(I've seen this in another clue)
'iron deficiency at the outset' is the wordplay.
'iron' becomes 'Fe' (chemical symbol for iron).
'at the outset' says to take the initial letters (the outset is the beginning).
The first letter of 'deficiency' is 'd'.
'fe'+'d'='FED'
(Other definitions for fed that I've seen before include "Dished up food" , "American cop" , "Nourished" , "Gave food to (someone)" , "US agent" .)