Terrible source of distraction, say (5)
I believe the answer is:
dread
'terrible' is the definition.
(I know that terrible can be written as dread)
'source of distraction say' is the wordplay.
'source of' suggests taking the first letters (the start - or source - of the word).
'say' becomes 'read' (synonyms).
The first letter of 'distraction' is 'd'.
'd'+'read'='DREAD'
(Other definitions for dread that I've seen before include "Deep fear" , "Dared to fear greatly" , "Great fear or apprehension" , "alarm" , "Enormous apprehension" .)