Toughened by rain? (9)
I believe the answer is:
weathered
'toughened' is the definition.
(similar in meaning)
'rain?' is the wordplay.
I cannot really understand how this works, but
'rain?' could be 'weather' (rain is a kind of weather) and 'weather' is located in the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'by' acts as a link.
Can you help me to learn more?
(Other definitions for weathered that I've seen before include "Worn by wind and rain" , "Strengthened by being exposed to the elements" .)