Seek new skill? Picked up skill, staying in control (7)
I believe the answer is:
'seek new skill?' is the definition.
(to retrain is to learn a new skill)
'picked up skill staying in control' is the wordplay.
'picked up' says the letters should be written in reverse.
'skill' becomes 'art' (both can mean a talent).
'staying in' indicates putting letters inside.
'control' becomes 'rein' (reining is a kind of controlling).
'art' reversed gives 'tra'.
'tra' put inside 'rein' is 'RETRAIN'.
(Other definitions for retrain that I've seen before include "Prepare for new job" , "Take refresher course" , "Teach skills for a new job" , "Acquire new skills" , "Terrain (anag.)" .)