Unique earlier power cable is relaid (13)
I believe the answer is:
irreplaceable
'unique' is the definition.
(I've seen this before)
'earlier power cable is relaid' is the wordplay.
'power' becomes 'p' (symbol in physics).
'is relaid' is an anagram indicator (letters laid again).
'earlier'+'p'+'cable'='earlierpcable'
'earlierpcable' with letters rearranged gives 'IRREPLACEABLE'.
(Other definitions for irreplaceable that I've seen before include "Without possible substitute" , "precious!" , "Unique" , "Cannot be substituted" , "Whose loss cannot be made good" .)