Some black cab always around, so taken unexpectedly (5)
I believe the answer is:
aback
'so taken unexpectedly' is the definition.
(as in being taken aback)
'some black cab always around' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'around' is a reversal indicator.
'kcaba' is hidden within 'black cab always'.
'kcaba' written backwards gives 'ABACK'.
(Other definitions for aback that I've seen before include "To take someone -- is to surprise them" , "impeded by headwind?" , "One is surprised when taken thus" , "(Taken) by surprise" , "Towards the rear (arch.)" .)