Small horse ridden by would-be queen more subject to depression? (9)

'more subject to depression?' is the definition.
(more vulnerable to being squashed or depressed)

'small horse ridden by would-be queen' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'horse' becomes 'h' (synonyms).
'ridden by' indicates putting letters inside (some letters ride on or around others).
'would-be' becomes 'quasi' (quasi can mean almost or would-be).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'h' inserted inside 'quasi' is 'quashi'.
's'+'quashi'+'er'='SQUASHIER'