Held back, being shy (8)
I believe the answer is:
reserved
I believe this clue is a double definition.
'held back' is the first definition.
(thesaurus)
'shy' is the second definition.
(reserved can mean shy or timid)
'being' is the link.
(Other definitions for reserved that I've seen before include "Booked - reticent" , "Booked, being unemotional" , "Held back, set aside" , "Set aside, earmarked" , "Shy; booked" .)