Difficulties with small horses (5)
I believe the answer is:
snags
'difficulties' is the definition.
(I know this)
'with small horses' is the wordplay.
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'horses' becomes 'nags' (nag is a kind of horse).
's'+'nags' is 'SNAGS'.
(Other definitions for snags that I've seen before include "Unexpected obstacles" , "Unforeseen obstacles" , "Problems" , "Hitches" , "Drawbacks" .)