It forms a strong personal attachment (6)
I believe the answer is:
tendon
'a strong personal attachment' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'it' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'endon' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'forms' acts as a link.
Can you help me to learn more?
(Other definitions for tendon that I've seen before include "Strong cord of tissue connecting muscle" , "Sinew, maybe 3" , "Fibrous band" , "Strong band of tissue connecting muscles" , "Connective tissue" .)