Can I use Melmetal's "Metal Eater" Ability to discard a {M} Pokemon from my hand if Melmetal is not damaged?
Yes you can. The first part of Metal Eater is to discard a {M} Pokemon from your hand, period. Then you do as much of the rest of the Ability as you are able to.
Source: Unbroken Bonds FAQ (2019-04-19), TPCi Rules Team (2019-05-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM10_129
SM10_129
SMP_SM181
SMP_SM181