Sensible to capture pencil marking on part of avian skeleton (8)
I believe the answer is:
wishbone
'part of avian skeleton' is the definition.
(I know this)
'sensible to capture pencil marking on' is the wordplay.
'sensible' becomes 'wise' (I've seen this in another clue).
'to capture' is an insertion indicator.
'pencil marking' becomes 'HB' (as in an HB pencil).
'hb'+'on'='hbon'
'wise' going around 'hbon' is 'WISHBONE'.
(Other definitions for wishbone that I've seen before include "Part of chicken" , "Bird-part pulled by two people for luck" , "V-shaped structure in chicken, turkey etc" , "Part of bird's skeleton" , "Furcula" .)