Nearby horse sound lacked energy (4)
I believe the answer is:
nigh
'nearby' is the definition.
(I've seen this before)
'horse sound lacked energy' is the wordplay.
'horse sound' becomes 'neigh' (I've seen this before).
'lacked' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'neigh' with 'e' removed is 'NIGH'.
(Other definitions for nigh that I've seen before include "Almost (arch.)" , "Adjacent" , "Just round the corner" , "Nearby" , "Near in time or place" .)