Whatever's keeping Louis X quaking with nerves? (9)
I believe the answer is:
anxiously
'with nerves?' is the definition.
'whatever's keeping louis x quaking' is the wordplay.
'whatever' becomes 'any'.
'keeping' is an insertion indicator.
'quaking' indicates an anagram (the letters quake or shake).
'louis'+'x'='louisx'
'louisx' with letters rearranged gives 'xiousl'.
'any' enclosing 'xiousl' is 'ANXIOUSLY'.
(Other definitions for anxiously that I've seen before include "Worriedly" , "With apprehension" .)