Day one got into physical prime (8)
I believe the answer is:
cardinal
'prime' is the definition.
(I've seen this before)
'day one got into physical' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'one' becomes 'i' (Roman numeral).
'got into' means one lot of letters goes inside another.
'physical' becomes 'carnal' (similar in meaning).
'd'+'i'='di'
'di' put within 'carnal' is 'CARDINAL'.
(Other definitions for cardinal that I've seen before include "Possibly one" , "Senior Roman Catholic" , "Senior churchman - bird" , "of prime importance" , "High-ranking official of the Roman Catholic church" .)