Attaching top rail inside is arduous (6)
I believe the answer is:
trying
'arduous' is the definition.
(thesaurus)
'attaching top rail inside' is the wordplay.
I cannot really see how this works, but
'inside' could be 'in' and 'in' is located in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for trying that I've seen before include "responsibility of the first Baron Jeffreys of Wem" , "particularly awkward" , "Making attempts" , "work on bench" , "Working in a court can be" .)