Clue: He dealt poorly with non-monogamous behaviour, being lazy (8,3,4)
Answer: SWINGING THE LEAD
Explanation: Definition: being lazy
=nautical slang referring to lead-weighted ropes used to gauge the depth of water.
 Brewer's gives: "an allusion to the lazy leadsman idly swinging the line and protracting the job of taking soundings"(he dealt)* after **SWINGING**="non-monogamous behaviour"