American man and sister hiding love for employer? (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A (abbreviation for American) + GENT (abbreviation for gentleman = man) + NUN (sister), containing O (zero = love in tennis scoring). Definition by example. In linguistics, an agent noun is a noun derived from some other word indicating an action, and means a person or thing who does that action. So from the verb “employ” we get the agent noun “employer” meaning a person who employs.