As a defender, never underlead an Ace against a suit contract.
Why not?
If dummy goes down on the table with a singleton King in that suit, your Ace will not win a trick and an Ace’s job is to kill a King or other high card.
Even worse, it might be declarer who has the singleton King so you won’t get the bad news until the second round of that suit. You would be very lucky to find your partner with the King so don’t risk it.
Your partner will never place you with the Ace in that suit …as s/he knows you should never underlead an Ace against a suit contract!
‘Underleading an Ace’ is when you have a suit containing the Ace and you lead any card other than the Ace. It is also sometimes referred to as ‘leading away from an Ace’.
This is only when defending against suit contracts, not NoTrumps.