City trimmed part of garden to do post-retirement activity (4,2,3)

'do post-retirement activity' is the definition.
(read after retiring to bed)

'city trimmed part of garden' is the wordplay.
'city' becomes 'Reading' (English city).
'trimmed' means to remove the last letter (the final letter is trimmed off).
'part of garden' becomes 'bed' (flower bed).
'reading' with its final letter taken off is 'readin'.
'readin'+'bed'='READ IN BED'

'to' is the link.