Attentiveness to strength (13)
I believe the answer is:
concentration
I believe this is a double definition.
'attentiveness' is the first definition.
(I have seen 'Complete attentiveness' mean 'concentration' so perhaps 'attentiveness' could also mean 'concentration')
'strength' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for concentration that I've seen before include "relative amount put in to get solution?" , "Absorption" , "Focussing" , "Deep thought" , "A close gathering of things" .)