Imagine leading when in second place (7)
I believe the answer is:
suppose
'imagine' is the definition.
('suppose' can be a synonym of 'imagine')
'leading when in second place' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'when in' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'place' becomes 'pose' (posing is a kind of placing**).
's'+'pose'='spose'
'up' put within 'spose' is 'SUPPOSE'.
(Other definitions for suppose that I've seen before include "Put forward - imagine" , "Presume" , "Expect" , "Assume, imagine" , "Hypothesize" .)