I must encounter resistance: a big blow (4)
I believe the answer is:
oner
'a big blow' is the definition.
(I've seen this before)
'i must encounter resistance' is the wordplay.
I cannot really see how this works, but
'i' could be 'one' (Roman numeral for one) and 'one' is found in the answer.
'resistance' could be 'r' (electronics abbreviation) and 'r' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oner that I've seen before include "Heavy blow; 1 note" , "Out-of-date banknote" , "A PS1 note (colloq.)" , "Big lie; unique thing" , "Outstanding person" .)