Dose of Aspirin, for example, efficient in races (6)
I believe the answer is:
Here is my best explanation:
'dose' is the definition.
(tablet is a kind of dose)
'efficient in races' is the wordplay.
'efficient' becomes 'able' (I've seen this before).
'in' is an insertion indicator.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'able' inserted into 'tt' is 'TABLET'.
'of aspirin for example' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.