Fire is strong, then weak (5)
I believe the answer is:
flame
'fire' is the definition.
(synonyms)
'strong then weak' is the wordplay.
'strong' becomes 'f' (musical symbol for 'forte', 'strong' in Italian).
'then' means one lot of letters go next to another.
'weak' becomes 'lame' (similar in meaning).
'f'+'lame'='FLAME'
'is' acts as a link.
(Other definitions for flame that I've seen before include "Flickering tongue of fire" , "Evidence of fire" , "Ignited gas" , "object of one's affection" , "Glowing part of fire" .)