As well it’s where a countdown is heading (3)
I believe the answer is:
too
'as well' is the definition.
(both can mean 'also')
'it's where a countdown is heading' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'oo' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for too that I've seen before include "To a greater than desirable extent" , "Furthermore" , "What a pity!" , "As well, also" , "Excessively" .)