Aged with hint of spasms in the heart – the effect of a virus (4,4)

'effect of a virus' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'aged with hint of spasms in the heart' is the wordplay.
'aged' becomes 'old' (similar in meaning).
'with' is a charade indicator (letters next to each other).
'hint of' says to take the initial letters (the smallest possible amount of the word).
'in the' is an insertion indicator.
'heart' becomes 'core' (synonyms).
The first letter of 'spasms' is 's'.
'old'+'s'='olds'
'olds' going within 'core' is 'COLD SORE'.

'the' is the link.

Can you help me to learn more?