Fee paid in advance for trusty servant (8)
I believe the answer is:
retainer
This might be a double definition.
'fee paid in advance' is the first definition.
(I've seen this before)
'trusty servant' is the second definition.
'retainer' can be an answer for 'servant' (synonyms). I am not sure about the 'trusty' bit.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for retainer that I've seen before include "Holding fee or faithful old servant" , "money to be paid in advance" , "Long-standing family servant" , "Long-serving member of staff" , "Household servant (often faithful and old!)" .)