Contact detective finally in attempt to reveal infidelity (9)
I believe the answer is:
treachery
'infidelity' is the definition.
(treachery is a kind of infidelity)
'contact detective finally in attempt' is the wordplay.
'contact' becomes 'reach' (synonyms).
'finally' suggests the final letters.
'in' means one lot of letters goes inside another.
'attempt' becomes 'try' (synonyms).
The last letter of 'detective' is 'e'.
'reach'+'e'='reache'
'reache' put inside 'try' is 'TREACHERY'.
'to reveal' acts as a link.
(Other definitions for treachery that I've seen before include "disloyalty" , "Two-timing" , "Perfidious behaviour" , "Betrayal of trust" , "Deceptiveness" .)