Negotiated mobile pact that's well suited (10)
I believe the answer is:
compatible
'well suited' is the definition.
(I've seen this before)
'negotiated mobile pact' is the wordplay.
'negotiated' indicates anagramming the letters (negotiate can mean to organise or arrange).
'mobile'+'pact'='mobilepact'
'mobilepact' is an anagram of 'COMPATIBLE'.
'that's' is the link.
(Other definitions for compatible that I've seen before include "Well suited" , "Able to coexist happily" , "in harmony" .)