Repeatedly cold, this most unconventionally fine shower! (6,4)
I believe the answer is:
scotch mist
'fine shower' is the definition.
(I've seen this before)
'repeatedly cold this most unconventionally' is the wordplay.
'repeatedly cold' means two substitutions for 'cold'.
'cold' becomes 'c' (eg on taps).
'cold' becomes 'c' (eg on taps).
'unconventionally' indicates an anagram.
'c'+'c'+'this'+'most'='ccthismost'
'ccthismost' anagrammed gives 'SCOTCH MIST'.
(Other definitions for scotch mist that I've seen before include "Very fine drizzle" , "Fine rain - something unsubstantial" , "fine shower" , "Highland weather" , "Thick Highland drizzle" .)