Christopher takes up net for for small pet (6)
I believe the answer is:
kitten
'pet' is the definition.
(I know that Kitten is a type of cat)
'christopher takes up net for for small' is the wordplay.
I cannot really understand how this works, but
'christopher' could be 'kit' (short for Christopher) and 'kit' is present in the answer.
an anagram of 'net' is 'ten' which is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for kitten that I've seen before include "one to become queen?" , "Baby cat" , "Baby rabbit" , "Little mewing ball of fur?" , "Young feline animal" .)