Settlement apt to be tricky with unknown fellows invading (7)
I believe the answer is:
payment
'settlement' is the definition.
(thesaurus)
'apt to be tricky with unknown fellows invading' is the wordplay.
'to be tricky with' is an anagram indicator.
'unknown' becomes 'y' (algebra).
'fellows' becomes 'men' (I've seen this before).
'invading' means one lot of letters goes inside another.
'apt' anagrammed with 'y' makes 'payt'.
'payt' going around 'men' is 'PAYMENT'.
(Other definitions for payment that I've seen before include "Reward for work" , "Recompense, reward" , "Money (for goods)" , "Settlement, remittance" , "Sum of money handed over" .)