Welcome seeing nightmare over (5)
I believe the answer is:
hello
'welcome' is the definition.
(both are greetings)
'nightmare over' is the wordplay.
'nightmare' becomes 'hell'.
'over' becomes 'o' (cricket abbreviation).
'hell'+'o'='HELLO'
'seeing' is the link.
(Other definitions for hello that I've seen before include "A salutation" , "Look at that" , "opener?" , "Conventional word of greeting" , "glossy magazine" .)