So he and I form a strong attachment (8)
I believe the answer is:
adhesion
'a strong attachment' is the definition.
(I've seen this before)
'so he and i form' is the wordplay.
'form' indicates anagramming the letters (a different form of the same letters).
'so'+'he'+'and'+'i'='soheandi'
'soheandi' is an anagram of 'ADHESION'.
(Other definitions for adhesion that I've seen before include "steady attachment" , "Grip" , "Stickiness" , "Tackiness" , "The property of sticking" .)