Moment one monster and knight seen in land? (3,4)
I believe the answer is:
big bang
'moment one' is the definition.
(first moment of the universe)
'monster and knight seen in land?' is the wordplay.
'monster' becomes 'big'.
'and' means one lot of letters go next to another.
'knight' becomes 'N' (chess abbreviation).
'seen in' is an insertion indicator.
'land?' becomes 'bag' (I've seen this in other clues).
'n' going into 'bag' is 'bang'.
'big'+'bang'='BIG BANG'
(Other definitions for big bang that I've seen before include "Universe's birth?" , "beginning to matter?" , "Theoretical origin of the universe" , "initiation theory?" , "major report" .)