Nervous swallows primarily twitter on the fence (7)
I believe the answer is:
neutral
'fence' is the definition.
(thesaurus)
'nervous swallows primarily twitter on the' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'nervous' could be 'neural' (synonyms) and 'neural' is found in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for neutral that I've seen before include "Lacking a preference" , "Gear position" , "Impartial; beige" , "having no potential" , "Not partisan or supporting either side in dispute" .)