Like this top firm (5)
I believe the answer is:
solid
'firm' is the definition.
(synonyms)
'like this top' is the wordplay.
'like this' becomes 'so' ('like so').
'top' becomes 'lid' (lid is a kind of top).
'so'+'lid'='SOLID'
(Other definitions for solid that I've seen before include "Concrete or reilable" , "Reliable - substantial - pure" , "Firm - sound - reliable" , "Reliable; not hollow" , "Rock-hard" .)