Source of protein, iron? Thanks! (4)
I believe the answer is:
feta
'source of protein' is the definition.
The definition suggests a singular noun which matches the answer.
'iron? thanks' is the wordplay.
'iron?' becomes 'Fe' (chemical symbol for iron).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'fe'+'ta'='FETA'
Can you help me to learn more?
(Other definitions for feta that I've seen before include "Greek salad cheese" , "Eastern Mediterranean cheese" , "A ewe's, goat's milk cheese" , "Goat's or sheep's milk cheese" , "Salty goat or sheep milk cheese" .)