Like some humour the writer's penned, surprisingly strong? (4)
I believe the answer is:
wiry
'surprisingly strong?' is the definition.
(wiry can mean sinewy or tough)
'like some humour the writer's penned' is the wordplay.
'like some humour' becomes 'wry' (wry humour is a type of humour).
'the writer' becomes 'I' (the person who is writing these clues).
'penned' indicates putting letters inside (inserted letters are enclosed or penned in).
'wry' going around 'i' is 'WIRY'.
(Other definitions for wiry that I've seen before include "Slender but strong and agile" , "strong and slim" , "Strong and flexible" , "with coarse hair" , "Lean, tough, sinewy" .)