Overbalanced head first! (7)

I believe the answer is:

toppled

I'm a little stuck... Click here to teach me more about this clue!

'overbalanced' is the definition.
(I've seen this before)

'head first' is the wordplay.
'head' becomes 'top' (synonyms).
'first' becomes 'pled' (I can't justify this - if you can you should believe this answer much more).
'top'+'pled'='TOPPLED'

Can you help me to learn more?

(Other definitions for toppled that I've seen before include "Brought down" , "Fell as if top-heavy" , "Ousted" , "Tipped over" , "Keeled over" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app