Twice per annum father is seen (4)
I believe the answer is:
papa
'father is seen' is the definition.
I can't tell whether this defines the answer.
'twice per annum' is the wordplay.
'twice' indicates the doubling of some letters.
'per annum' becomes 'p.a.' (synonyms).
'pa' duplicated is 'PAPA'.
Can you help me to learn more?
(Other definitions for papa that I've seen before include "Father; letter P" , "Pop" , "The old man" , "Comms code for 'P'" , "Daddy]" .)