Unclued lights (three of two words; ignore one apostrophe) may be grouped to form a related triad.
 

Across

 
1    Old wife to claim freebie from party (8, two words)
5    Arrest of secret police (6)
11    King’s job reduced: that hurt part of the fleet (10)
16    From lack of energy Roman general loses a navy (5)
17    Cudgels seen abandoned on mountain (7)
20    Special toy for child with which to paint (8)
25    From poet, a conceit for 28 Feb? (4)
26    Electronic signal about work for shepherdess (6, hyphened)
28    Pay back cleric in time (6)
29    With difficulty, breathe air quietly (4)
31    Such a successful man feels...