Less than solid (4)
I believe the answer is:
iffy
'less than solid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iffy that I've seen before include "Informally dubious or uncertain" , "Informally dubious, not definite" , "Full of uncertainty" , "Problematic" , "Not to be trusted" .)