Time relative backed strange patent medicine (7)
I believe the answer is:
nostrum
'patent medicine' is the definition.
(nostrum is a kind of patent medicine)
'time relative backed strange' is the wordplay.
'time' becomes 't' (abbreviation).
'relative' becomes 'son' (son is a type of relative').
'backed' shows that the letters should be reversed in order.
'strange' becomes 'rum' (synonyms).
't'+'son'='tson'
'tson' back-to-front is 'nost'.
'nost'+'rum'='NOSTRUM'
(Other definitions for nostrum that I've seen before include "Must Ron take a quack remedy?" , "Hypothetical remedy for all ills" , "Simple, remedy" , "Patent medicine of highly questionable value" , "Some think it helps" .)