Get behind trees overlooking motorway briefly and peer, mostly unseen (12)

'peer mostly unseen' is the definition.
(a peer who rarely attends the House of Lords)

'get behind trees overlooking motorway briefly and' is the wordplay.
'get behind' becomes 'back' (back can mean to get behind or support).
'trees' becomes 'woods' (as in a forest).
'overlooking' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'motorway' becomes 'M' (as in motorway names - Mw, Md etc).
'briefly' means to remove the last letter.
'and' with its last letter removed is 'an'.
'back'+'woods'+'m'+'an'='BACKWOODSMAN'