Some trouble afoot? Maybe one's fallen in the wood (4)
I believe the answer is:
leaf
'maybe one's fallen in the wood' is the definition.
(a leaf may fall in a wood)
'some trouble afoot?' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'LEAF' can be found hidden inside 'trouble afoot'.
(Other definitions for leaf that I've seen before include "Page - part of a plant" , "See 1" , "Foil" , "Single sheet of a book" , "Organ of photosynthesis in plants" .)