Period given to R.N. formation to enter battle (9)
I believe the answer is:
fortnight
'period' is the definition.
(fortnight is a kind of period)
'to r n formation to enter battle' is the wordplay.
'formation' is an anagram indicator.
'to enter' means one lot of letters goes inside another.
'battle' becomes 'fight' (fight is a kind of battle**).
'to'+'r'+'n'='torn'
'torn' anagrammed gives 'ortn'.
'ortn' inserted into 'fight' is 'FORTNIGHT'.
'given' acts as a link.
(Other definitions for fortnight that I've seen before include "Half a lunar month" , "Fourteen days" , "Period of time" , "couple of weeks" , "Two weeks" .)