He is one having long shackle (7)
I believe the answer is:
manacle
'shackle' is the definition.
(manacle is a kind of shackle)
'he is one having long' is the wordplay.
'he is' becomes 'man' (I am not sure about this - if you are sure you should believe this answer much more).
'one' becomes 'ace' (synonyms).
'having' is an insertion indicator.
'long' becomes 'l'.
'ace' going around 'l' is 'acle'.
'man'+'acle'='MANACLE'
Can you help me to learn more?
(Other definitions for manacle that I've seen before include "Shackle on the wrist" , "item restricting movement" , "Curb" , "Metal shackle" , "restricting at the cuff?" .)