Fit into Dot's building, otherwise it can stop the horses bolting (6,4)
I believe the answer is:
stable door
'it can stop the horses bolting' is the definition.
(horses might bolt through a stable door)
'fit into dot's building otherwise' is the wordplay.
'fit' becomes 'able' (similar in meaning).
'into' indicates putting letters inside.
'building' indicates an anagram.
'otherwise' becomes 'or'.
'dots' with letters rearranged gives 'stdo'.
'able' placed into 'stdo' is 'stabledo'.
'stabledo'+'or'='STABLE DOOR'
(Another definition for stable door that I've seen is " It's often locked after the horse has bolted".)