Is within twelve inches (thus handy?) (4)
I believe the answer is:
fist
'inches thus handy?' is the definition.
I can't tell whether this definition defines the answer.
'is within twelve' is the wordplay.
'within' means one lot of letters goes inside another.
'twelve' becomes 'ft' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is' going within 'ft' is 'FIST'.
Can you help me to learn more?
(Other definitions for fist that I've seen before include "Tightly-closed hand" , "Hit with a clenched one" , "One may deck in a ring" , "Punch dispenser" , "duke" .)