A 'Special One' helping to produce body fluid (12)
I believe the answer is:
perspiration
'body fluid' is the definition.
(I know that perspiration is a type of secretion)
'a special one helping' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'special' becomes 'sp' (abbreviation).
'one' becomes 'i' (Roman numeral).
'helping' becomes 'ration' (a ration is a serving or helping).
'per'+'sp'+'i'+'ration'='PERSPIRATION'
'to produce' acts as a link.
(Other definitions for perspiration that I've seen before include "Sweat" , "is exuded" , "Ninety nine percent of genius" .)