Mobile north side of city becomes troublesome (6)
I believe the answer is:
thorny
'troublesome' is the definition.
(I've seen this before)
'mobile north side of city' is the wordplay.
'mobile' indicates anagramming the letters.
'side of' suggests the final letters.
The last letter of 'city' is 'y'.
'north' is an anagram of 'thorn'.
'thorn'+'y'='THORNY'
'becomes' acts as a link.
(Other definitions for thorny that I've seen before include "(Of problem) causing difficulty; prickly" , "Problematic; having spines" , "Prickly, problematic" , "Spiny - difficult and controversial" , "Covered in spikes" .)