Sentence:  Even the Pilot's Operating Handbooks (POH) for our training airplanes add to our paranoia with their insistence that we not <WSD>lean</WSD> the mixture until we're above 5000 feet density altitude. 
Meaning of the word marked with <WSD> tags:  To thin out (a fuel-air mixture): to reduce the fuel flow into the mixture so that there is more air or oxygen.