Word of prayer when beset by minor physical disability (8)

'physical disability' is the definition.
(inability to walk or difficulty walking)

'word of prayer when beset by minor' is the wordplay.
'word of prayer' becomes 'amen' (last word in a prayer).
'when beset by' is an insertion indicator (beset can mean to hem in or enclose).
'minor' becomes 'less' (I've seen this before).
'amen' inserted within 'less' is 'LAMENESS'.