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' indicates putting letters inside (some letters go outside others).
'all' placed into 'sows' is 'SALLOWS'.

'producing' is the link.