Condensation in front headlight

[smiley08]

Guys tried searching for this but it didn't bring up any results, so apologies if this has been asked a million times before. I got a few lightbulbs replaced by mechanic an then my headlight has started to 'steam' up an also theres a fly n possibly fly sh*t on the inside of the cover. The mechanic never put the cover on properly and it's fallen off so i think this is the source of the problem. is there anyway of cleaning this so it doesn't happen anymore?