Man, perhaps Dennis, cycles after setter (4)
I believe the answer is:
isle
'man perhaps' is the definition.
(Isle of Man is an example)
'dennis cycles after setter' is the wordplay.
'dennis cycles' becomes 'sle' (I can't justify this - if you can you should give a lot more credence to this answer).
'after' means one lot of letters go next to another.
'setter' becomes 'I' (the person who set this crossword).
'sle' after 'i' is 'ISLE'.
Can you help me to learn more?
(Other definitions for isle that I've seen before include "one ringed in the main?" , "one has water all round" , "See 10 Down" , "Skye or Capri?" , "Body of land surrounded by water" .)