Loyal woman developed flu (8)
I believe the answer is:
faithful
'loyal' is the definition.
('faithful' can be similar in meaning to 'loyal')
'woman developed flu' is the wordplay.
'woman' becomes 'faith' (**).
'developed' indicates an anagram.
'flu' with letters rearranged gives 'ful'.
'faith'+'ful'='FAITHFUL'
(Other definitions for faithful that I've seen before include "Steadfast and true" , "Constant and true" , "Trustworthy and true" , "Loyal and true" , "Steadfast in affection or allegiance" .)