Strong; reliable (5)
I believe the answer is:
solid
This clue is a double definition.
'strong' is the first definition.
(similar in meaning)
'reliable' is the second definition.
(I've seen this before)
(Other definitions for solid that I've seen before include "Of substance" , "Reliable - strong" , "Firm - sound - reliable" , "Shape with three dimensions" , "Reliable - substantial - pure" .)