Lost, dear - upsetting for the guide (8)
I believe the answer is:
lodestar
'the guide' is the definition.
(I've seen this before)
'lost dear upsetting' is the wordplay.
'upsetting' indicates anagramming the letters.
'lost'+'dear'='lostdear'
'lostdear' with letters rearranged gives 'LODESTAR'.
'for' is the link.
(Other definitions for lodestar that I've seen before include "In navigation, celestial reference point" , "Guiding principle" , "Heavenly guide" , "Travel guide" , "Guiding light in sky" .)