Truss half a reptile? (3)
I believe the answer is:
liz
'truss' is the definition.
(UK politician Liz Truss)
'half a reptile?' is the wordplay.
'half a' indicates taking half.
'reptile?' becomes 'lizard' (lizard is a kind of reptile).
'lizard' with half the letters taken is 'LIZ'.
(Other definitions for liz that I've seen before include "Elizabeth (dim.)" , "Taylor or Hurley?" , "Short for Elizabeth" .)