Old relative's ignoring large cats (6)
I believe the answer is:
ounces
'large cats' is the definition.
'ounces' can be an answer for 'cats' (I have seen 'Undomesticated cats' mean 'ounces' so perhaps 'cats' could also mean 'ounces'). I'm not certain of the 'large' bit.
'old relative's ignoring' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'relative's ignoring' becomes 'unces' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o'+'unces'='OUNCES'
Can you help me to learn more?
(Other definitions for ounces that I've seen before include "More than one cat" , "Snow leopards - 16 of them in pound" , "There are16 to the pound" , "Light weights or very small amounts" , "Small weights in UNESCO" .)