Four-footers balance unsteadily, suspicious at first around vehicle (11)

'four-footers' is the definition.
(lines of poetry containing four metric feet)

'balance unsteadily suspicious at first around vehicle' is the wordplay.
'balance unsteadily' becomes 'teeter' (I've seen this before).
'suspicious at first' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'around' is an insertion indicator.
'vehicle' becomes 'tram' (tram is a kind of vehicle).
'teeter'+'s'='teeters'
'teeters' enclosing 'tram' is 'TETRAMETERS'.

Can you help me to learn more?