A bundle of surprising letters left on shelf (6)
I believe the answer is:
single
'left on shelf' is the definition.
(informal term for romantically single)
'a bundle of surprising letters' is the wordplay.
'a bundle of' says the answer is hidden in the clue.
'SINGLE' is hidden in the letters of 'surprising letters'.
(Other definitions for single that I've seen before include "Not married, just the one" , "Solo" , "Like bachelor" , "Still miss" , "A hit for one run in cricket" .)