Old man backed intention to put in post (7)
I believe the answer is:
appoint
'put in post' is the definition.
(as in appointing someone to a post)
'old man backed intention' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'backed' is a reversal indicator.
'intention' becomes 'point'.
'pa' in reverse letter order is 'ap'.
'ap'+'point'='APPOINT'
'to' is the link.
(Other definitions for appoint that I've seen before include "Assign to a duty" , "get the post sorted" , "Assign job to (someone)" , "Choose for a job" , "Give job to" .)