Fortify; improve (6)
I believe the answer is:
enrich
This clue is a double definition.
'fortify' is the first definition.
(I've seen this in another clue)
'improve' is the second definition.
(I know that enrich is a more specific form of the action improve)
(Other definitions for enrich that I've seen before include "Make fuller flavoured" , "increase wealth" , "Make wealthy or improve the quality" , "Heighten" , "Adorn, enhance or make wealthy" .)