Old settler's call on husband to conserve energy (8)
I believe the answer is:
visigoth
'old settler's' is the definition.
(the Visigoths settled in Spain)
'call on husband to conserve energy' is the wordplay.
'call on' becomes 'visit' (synonyms).
'husband' becomes 'h' (abbreviation).
'to conserve' means one lot of letters goes inside another.
'energy' becomes 'go' (go can mean energy or enthusiasm).
'visit'+'h'='visith'
'visith' enclosing 'go' is 'VISIGOTH'.
(Other definitions for visigoth that I've seen before include "Old European" , "Roman Empire invader" , "he had a smashing time in Rome" , "old Germanic type" , "Alaric, eg, who conquered Rome" .)