Glance up to see part of castle (4)
I believe the answer is:
keep
'part of castle' is the definition.
('keep' is part of 'castle')
'glance up' is the wordplay.
'glance' becomes 'peek' (I have seen 'Furtive glance ' mean 'peek' so perhaps 'glance' could also mean 'peek').
'up' says the letters should be written backwards (in down clue: letters go upwards).
'peek' back-to-front is 'KEEP'.
'to see' acts as a link.
(Other definitions for keep that I've seen before include "Main tower of a medieval castle" , "Hold - part of castle" , "Only one cricketer on field will" , "Retain, maintain" , "Board and lodgings, perhaps" .)