Detective longed to be apart (8)
I believe the answer is:
detached
'be apart' is the definition.
(synonyms)
'detective longed' is the wordplay.
'detective' becomes 'det' (abbreviation for detective).
'longed' becomes 'ached' ('ache' can be a synonym of 'long').
'det'+'ached'='DETACHED'
'to' is the link.
(Other definitions for detached that I've seen before include "Separated from - maybe a house" , "cut off" , "Type of house" , "isolated" , "Not committed, objective" .)