Vault tree, avoiding lake (4)
I believe the answer is:
arch
'vault' is the definition.
(thesaurus)
'tree avoiding lake' is the wordplay.
'tree' becomes 'larch' (larch tree is a kind of tree).
'avoiding' indicates named letters should be taken away.
'lake' becomes 'l'.
'larch' with 'l' taken out is 'ARCH'.
(Other definitions for arch that I've seen before include "Curved part of stone bridge, perhaps" , "Curved vault" , "Curve spanning an opening" , "Curved masonry construction" , "Foremost" .)