Old boy and girl taking over American patent (7)
I believe the answer is:
obvious
'patent' is the definition.
(similar in meaning)
'old boy and girl taking over american' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'and' says to put letters next to each other.
'girl' becomes 'Vi' (short for Violet).
'taking over' becomes 'o' (cricket abbreviation. I am not sure about the 'taking' bit.).
'american' becomes 'us' (abbreviation for United States).
'ob'+'vi'+'o'+'us'='OBVIOUS'
(Other definitions for obvious that I've seen before include "Quite clear and manifest" , "Plain to be seen" , "Evident, plain" , "Clear; easy to see" , "Plainly seen" .)