Encountered a bit of resistance, at length (5)
I believe the answer is:
metre
'length' is the definition.
(I've seen this before)
'encountered a bit of resistance' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'a bit of resistance' becomes 're' (I can't explain this - if you can you should give a lot more credence to this answer).
'met'+'re'='METRE'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "Unit of length in metric system" , "Basic unit of length" , "100cm" , "Poetic rhythm" , "determined by scanning" .)