Sources of comfort only found lining fashionable shoes at first (7)

'sources of comfort' is the definition.
(items which make shoes more comfortable)

'only found lining fashionable shoes at first' is the wordplay.
'only' becomes 'sole' (synonyms).
'found lining' means one lot of letters goes inside another (as in something's contents or lining).
'fashionable' becomes 'in' (similar in meaning).
'at first' says to take the initial letters.
The initial letter of 'shoes' is 's'.
'in'+'s'='ins'
'sole' inserted into 'ins' is 'INSOLES'.