It's silly batting an eyelid to begin with (5)
I believe the answer is:
inane
'it's silly' is the definition.
(thesaurus)
'batting an eyelid to begin with' is the wordplay.
'batting' becomes 'in'.
'to begin with' indicates taking the first letters.
The first letter of 'eyelid' is 'e'.
'in'+'an'+'e'='INANE'
(Other definitions for inane that I've seen before include "Puerile" , "Lacking meaning" , "Witless and pointless" , "Fatuous, vacuous" , "Empty-headed" .)