Insects beneath organ -- flies, indefinite number ignored (7)
'insects' is the definition.
(earwig is a kind of insect)
'beneath organ flies indefinite number ignored' is the wordplay.
'beneath' means one lot of letters go next to another (in a down clue).
'organ' becomes 'ear' (ear is a kind of organ**).
'flies' becomes 'wings' (wing can mean to fly).
'indefinite number' becomes 'n' (I've seen this in other clues).
'ignored' indicates named letters should be taken away.
'wings' with 'n' taken out is 'wigs'.
'ear' put next to 'wigs' is 'EARWIGS'.
(Other definitions for earwigs that I've seen before include "Bugs", "Eavesdrops on the insects", "Insect with pincers", "Elongated insects", "listens in furtively".)