Fruit, often with a thorn on (3)
I believe the answer is:
haw
'fruit' is the definition.
(I've seen this before)
'often with a thorn on' is the wordplay.
I cannot really see how this works, but
'with' could be 'w' (abbreviation) and 'w' is found within the answer.
'a' is found in the answer.
A single letter 'h' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for haw that I've seen before include "Fruit of common thorn tree (3)" , "Fruit of the may tree" , "Fruit of the hawthorn" , "Hedgerow fruit" , "Berry of a thorny shrub" .)