Engage in experiment involving rattlesnake's skin (8)
I believe the answer is:
interest
'engage' is the definition.
(thesaurus)
'in experiment involving rattlesnake's skin' is the wordplay.
'experiment' becomes 'test' (test is a kind of experiment).
'involving' means one lot of letters goes inside another.
'skin' means to remove the middle letters.
'rattlesnake' with its middle removed is 're'.
'test' going around 're' is 'terest'.
'in'+'terest'='INTEREST'
(Other definitions for interest that I've seen before include "thing" , "something that is enjoyable" , "attention is shown" , "Money paid by borrower" , "Sum of money paid on top of borrowed amount" .)