A letter perused in fear (5)
I believe the answer is:
dread
'fear' is the definition.
(I know that dread is a type of fear)
'a letter perused' is the wordplay.
'a letter' becomes 'd' ().
'perused' becomes 'read' (I've seen this before).
'd'+'read'='DREAD'
'in' is the link.
(Other definitions for dread that I've seen before include "Trepidation" , "Be afraid of - anxiety" , "Fear - dire" , "Anticipate fearfully" , "Great fear or apprehension" .)