Two readings, two words. This one has a neutral second syllable and means the knack or know-how, as in 内行看门道，外行看热闹, experts see the technique while outsiders just see the show. With a full fourth tone, 门道 is the passage through a gateway.
usage
Very close to 门路, and both are colloquial. This word leans towards the inside technique that makes something work, while 门路 leans towards the connection or channel that gets you in.
