Brief lapse by one getting on metro is an omen, not a failing (6,6)
I believe the answer is:
senior moment
'brief lapse' is the definition.
The definition suggests a singular noun which matches the answer.
'one getting on metro is an omen not a failing' is the wordplay.
'one getting on' says to put letters next to each other (in a down clue, some letters go on top of others).
'not' is a deletion indicator.
'failing' indicates an anagram (a failed spelling of the word).
'metro'+'is'+'an'='metroisan'
'metroisan' after 'omen' is 'omenmetroisan'.
'omenmetroisan' with 'a' taken away is 'omenmetroisn'.
'omenmetroisn' with letters rearranged gives 'SENIOR MOMENT'.
'by' is the link.
Can you help me to learn more?
(Other definitions for senior moment that I've seen before include "memory lapse" , "Fleeting loss of memory" .)