Tissue in cart single male has filled (4,6)
I believe the answer is:
bone marrow
'tissue' is the definition.
(bone marrow is a kind of tissue)
'cart single male has filled' is the wordplay.
I cannot quite understand how this works, but
'single' could be 'one' (a single thing - only one) and 'one' is found in the answer.
'male' could be 'm' (common abbreviation) and 'm' is present in the answer.
'cart' could be 'barrow' (barrow is a kind of cart) and 'barrow' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for bone marrow that I've seen before include "Fatty network of connective tissue" , "transplant tissue" , "Soft tissue" , "Vital part of skeleton" , "Producer of blood cells" .)