Measure of speed: A little kayak gets flipping 100mph! (4)
I believe the answer is:
knot
'measure of speed' is the definition.
(nautical unit of speed)
'a little kayak gets flipping 100mph' is the wordplay.
'a little' suggests taking the first letters.
'gets' says to put letters next to each other.
'flipping' shows that the letters should be reversed in order.
'100mph' becomes 'ton' (I've seen this in other clues).
The initial letter of 'kayak' is 'k'.
'ton' reversed gives 'not'.
'k'+'not'='KNOT'
(Other definitions for knot that I've seen before include "Nautical speed - group of toads" , "Shore bird, tangle" , "Fastening - unit of speed" , "Bird; hard part in wood" , "Could be a granny or a sheepshank" .)