Once inside, I can't move and that's wrong (7)
I believe the answer is:
Here is my best explanation:
'wrong' is the definition.
'once inside i can't move' is the subsidiary indication.
'once' becomes 'ex'.
'inside' is an insertion indicator.
'move' is an anagram indicator.
'icant' is an anagram of 'inact'.
'ex' inserted into 'inact' is 'inexact'.
'and that's' is the link.
(Other definitions for inexact that I've seen before include "not wholly Right", "Rough", "Not quite right", "woolly", "Imprecise - inaccurate in", "Lax", "Not precise, approximate", "Nice tax (anag) - approximate", "Nice tax (nag.)" and "loose".)