Everyone plants seeds outside, producing trees (7)

'trees' is the definition.
(sallow is a kind of tree)

'everyone plants seeds outside' is the wordplay.
'everyone' becomes 'all' (all people).
'plants seeds' becomes 'sows' (I've seen this in another clue).
'outside' is an insertion indicator (some letters go outside others).
'all' going inside 'sows' is 'SALLOWS'.

'producing' acts as a link.