Weakling, say, losing heart, is initially strong inside (5)
I believe the answer is:
sissy
'weakling' is the definition.
(I've seen this before)
'say losing heart is initially strong inside' is the wordplay.
'losing heart' says to hollow out the word (remove centre letters).
'initially' indicates taking the first letters.
'inside' is an insertion indicator.
'say' with its middle removed is 'sy'.
The first letter of 'strong' is 's'.
'is'+'s'='iss'
'sy' placed around 'iss' is 'SISSY'.
(Other definitions for sissy that I've seen before include "Effeminate boy" , "Unmanly man" , "Effeminate and over-timorous male" , "Effeminate (colloq.)" , "Weakling" .)