Erect a mobile up in cradle (13)
I believe the answer is:
perpendicular
'erect' is the definition.
(synonyms)
'a mobile up in cradle' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'mobile' is an anagram indicator.
'up'+'in'+'cradle'='upincradle'
'upincradle' with letters rearranged gives 'pendicular'.
'per'+'pendicular'='PERPENDICULAR'
(Other definitions for perpendicular that I've seen before include "Like this answer [1A], relative to 1 dn" , "Vertical - at 90 degrees" , "style of construction" , "Sort of architecture" , "Sheer" .)