1.
If you let someone down, you disappoint them, by not doing something that you have said you will do or that they expected you to do.
Don’t worry, Xiao, I won’t let you down...
When such advisers fail in their duty, they ~ the whole system.
PHRASAL VERB: V n P, V P n (not pron)
~
The company now has a large number of workers who feel badly ~.
ADJ: v-link ADJ
2.
If something lets you down, it is the reason you are not as successful as you could have been.
Many believe it was his shyness and insecurity which let him down...
Sadly, the film is ~ by an excessively simple plot.
PHRASAL VERB: V n P, V n P, also V P n (not pron)
3.
If you ~ something such as a tyre, you allow air to escape from it. (mainly BRIT)
I let the tyres down on his car...
Remove wheelnuts, ~ tyre, put on spare.
= deflate
PHRASAL VERB: V n P, V P n (not pron)