Former sweetheart's love starts to look decidedly feeble and weak (3,5)
I believe the answer is:
old flame
'former sweetheart's' is the definition.
(I've seen this before)
'love starts to look decidedly feeble and weak' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'starts to' says to take the initial letters.
'and' means one lot of letters go next to another.
'weak' becomes 'lame' ('lame' can be similar in meaning to 'weak'**).
The initial letters of 'look decidedly feeble' is 'ldf'.
'o'+'ldf'+'lame'='OLD FLAME'
(Other definitions for old flame that I've seen before include "Former lover" , "Ex-lover - the fire still burns" , "Former sweetheart" , "Ex" , "still 1 across [ALIGHT], it seems" .)