Think as timid type that's lost love (4)
I believe the answer is:
muse
'think' is the definition.
(I've seen this before)
'timid type that's lost love' is the wordplay.
'timid type' becomes 'mouse' (I've seen this before).
'that's lost' is a deletion indicator.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'mouse' with 'o' taken out is 'MUSE'.
'as' is the link.
(Other definitions for muse that I've seen before include "The source of an artist's inspiration" , "Ponder on one of nine" , "Contemplate -- a goddess of the liberal arts" , "Cogitate" , "Contemplation" .)