Gathering a McLaren first overtaking after start (8)
I believe the answer is:
amassing
'gathering' is the definition.
('amass' can be a synonym of 'gather')
'a mclaren first overtaking after start' is the wordplay.
'mclaren first' becomes 'm' (1st letter of 'mclaren').
'overtaking' becomes 'passing' (synonyms).
'after start' means to remove the first letter (I've seen 'after starting' mean this (only the letters found after the first are used)).
'passing' with its initial letter removed is 'assing'.
'a'+'m'+'assing'='AMASSING'
(Other definitions for amassing that I've seen before include "Accumulating a large amount" , "Putting together" , "gathering" , "Making a collection of" , "Accumulating in quantity" .)