Mark is catty - suffering iron deficiency? (4)
I believe the answer is:
line
'mark' is the definition.
(lining is a kind of marking)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for line that I've seen before include "It's long relative to its width" , "Phone connection; queue" , "Policy - file" , "Family" , "column" .)