Annual percentage rate, one lowered initially for a month (5)
I believe the answer is:
april
'month' is the definition.
(I know that Gregorian calendar month is a type of month)
'annual percentage rate one lowered initially' is the wordplay.
'annual percentage rate' becomes 'APR' (abbreviation).
'one' becomes 'i' (Roman numeral).
'initially' indicates taking the first letters.
The initial letter of 'lowered' is 'l'.
'apr'+'i'+'l'='APRIL'
'for a' acts as a link.
(Other definitions for april that I've seen before include "Part of spring" , "'All in an ... evening' (Katharine Tynan)" , "Start of the financial year" , "period of time" , "Woman" .)