Substitute tough material with a flimsy one (7)
I believe the answer is:
replace
'substitute tough' is the definition.
'replace' can be an answer for 'substitute' (synonyms). I'm unsure of the 'tough' bit.
'material with a flimsy one' is the wordplay.
'material' becomes 'rep' (rep is a kind of material).
'with' means one lot of letters go next to another.
'a flimsy one' becomes 'lace' (I am not sure about this - if you are sure you should believe this answer much more).
'rep'+'lace'='REPLACE'
Can you help me to learn more?
(Other definitions for replace that I've seen before include "succeed" , "Put back where you found it" , "Substitute for (someone)" , "Put back or substitute" , "Put something back where it belongs" .)