Moving rapidly, or stuck when grabbed by foot (4)
I believe the answer is:
fast
'moving rapidly or stuck' is the definition.
I can't judge whether this defines the answer.
'when grabbed by foot' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'grabbed by' means one lot of letters goes inside another (inserted letters are grabbed inside).
'foot' becomes 'ft' (synonyms).
'as' inserted inside 'ft' is 'FAST'.
Can you help me to learn more?
(Other definitions for fast that I've seen before include "Don't scoff" , "Unlikely to run" , "eg Ramadan; speedy" , "Maybe bound to spot" , "Abstain from food/drink" .)