Poetry by Frost, perhaps, expressed apprehension (4,4)
I believe the answer is:
cold feet
'expressed apprehension' is the definition.
'cold feet' can be an answer for 'apprehension' (I've seen this before). I'm not sure about the 'expressed' bit.
'poetry by frost' is the wordplay.
'poetry' becomes 'feet' (poetic structures are made up of metric feet).
'by' is a charade indicator (letters next to each other).
'frost' becomes 'cold' (I can't explain this - if you can you should give a lot more credence to this answer).
'feet' after 'cold' is 'COLD FEET'.
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for cold feet that I've seen before include "Loss of courage or confidence" , "Icy extremities for slang fear" , "Lack of courage or confidence" , "result of paddling in winter ?" , "Feeling of fear - in lower extremities?" .)