Too frightened to utter a cry of pain (6)
I believe the answer is:
yellow
'too frightened' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives can occasionally define each other.
'utter a cry of pain' is the wordplay.
'utter' becomes 'yell' (yelling is a kind of uttering).
'a cry of pain' becomes 'ow' (exclamation of pain).
'yell'+'ow'='YELLOW'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for yellow that I've seen before include "Spineless" , "See 15" , "like a coward" , "like a canary" , "likely to run" .)